My favorite SSA problem

Last month, I had a series of posts on solving a triangle when two sides and a non-included angle are given. Here is my all-time favorite word problem along these lines:

Assume that Venus and Earth both have circular orbits around the sun with radii 68 million miles and 93 million miles, respectively. Just after sunrise, an astronomer sees Venus on the horizon and measures the angle between Venus and the sun to be 20 degrees. Find the possible distances from Venus to Earth at that moment.

venus

I won’t go through the solution of the problem… it’s a fairly straightforward application of SSA techniques. But I’ve always had a soft spot for this problem… probably because I have a soft spot for astronomy and the picture of the planets in their orbits makes perfectly clear why the information can narrow down the answer to two possible solutions, but more information is needed in order to figure out which one is actually correct.

Inverse functions: Arcsine and SSA (Part 17)

In the last few posts, we studied the SSA case of solving for a triangle, when two sides and an non-included angle are given. (Some mathematics instructors happily prefer the angle-side-side acronym to bluntly describe the complications that arise from this possibly ambiguous case. I personally prefer not to use this acronym.)

A note on notation: when solving for the parts of \triangle ABC, a will be the length of the side opposite \angle A, b will be the length of the side opposite \angle B, and c will be the length of the side opposite angle C. Also \alpha will be the measure of \angle A, \beta will be measure of \angle B, and \gamma will be the measure of \angle C. Modern textbooks tend not to use \alpha, \beta, and \gamma for these kinds of problems, for which I have only one response:https://meangreenmath.files.wordpress.com/2014/10/philistines.png

philistines

Suppose that a, c, and the nonincluded angle \alpha are given, and we are supposed to solve for b, \beta, and \gamma. As we’ve seen in this series, there are four distinct cases — and handling these cases requires accurately solving equation like \sin \gamma = \hbox{something} on the interval [0^\circ, 180^\circ].

Case 1. b < c \sin \alpha. In this case, there are no solutions. When the Law of Sines is employed and we reach the step

\sin \gamma = \hbox{something}

the \hbox{something} is greater than 1, which is impossible.

SSA1

Case 2. b = c \sin \alpha. This rarely arises in practice (except by careful writers of textbooks). In this case, there is exactly one solution. When the Law of Sines is employed, we obtain

\sin \gamma = 1

We conclude that \gamma = 90^\circ, so that \triangle ABC is a right triangle.

SSA2

Case 3. c \sin \alpha < b < c. This is the ambiguous case that yields two solutions. The Law of Sines yields

\sin \gamma = \hbox{something}

so that there are two possible choices for \gamma, \hbox{some angle} and 180^\circ - \hbox{some angle}.

SSA4

Case 4. b > c. This yields one solution. Similar to Case 3, the Law of Sines yields

\sin \gamma = \hbox{something}

so that there are two possible choices for \gamma, \hbox{some angle} and 180^\circ - \hbox{some angle}. However, when the second larger value of \gamma is attempted, we end up with a negative angle for \beta, which is impossible (unlike Case 3).

 

SSA3Many mathematics students prefer to memorize rules like those listed above. However, I try to encourage my students not to blindly use rules when solving the SSA case, as it’s just too easy to make a mistake in identifying the proper case. Instead, I encourage them to use the Law of Sines and to remember that the equation

\sin \gamma = t

has two solutions in [0^\circ, 180^\circ] as long as $0 < t < 1$:

\gamma = \sin^{-1} t \qquad \hbox{and} \qquad \gamma = 180^\circ - \sin^{-1} t

If they can remember this fact, then students can just follow their noses when applying the Law of Sines, identifying impossible and ambiguous cases when the occasions arise.

 

Inverse functions: Arcsine and SSA (Part 16)

We’ve seen in this series that blinding using the arcsine function on a calculator is insufficient for finding all solutions of an equation like \sin \theta = 0.8. In today’s post, I discuss one of the first places that this becomes practically important: solving the ambiguous case of solving a triangle given two sides and an nonincluded angle.

A note on notation: when solving for the parts of \triangle ABC, a will be the length of the side opposite \angle A, b will be the length of the side opposite \angle B, and c will be the length of the side opposite angle C. Also \alpha will be the measure of \angle A, \beta will be measure of \angle B, and \gamma will be the measure of \angle C. Modern textbooks tend not to use \alpha, \beta, and \gamma for these kinds of problems, for which I have only one response:

philistines

Why does an SSA triangle produce an ambiguous case (unlike the SAS, SSS, or ASA cases)? Here’s a possible problem that has two different solutions:

Solve \triangle ABC if a = 8, c = 10, and \alpha = 30^\circ.

A student new to the Law of Sines might naively start solving the problem by drawing something like this:

badSSA4

Of course, that’s an inaccurate picture that isn’t drawn to scale. A more accurate picture would look like this:

SSA4This time, the red circle intersects the dashed black line at two different points. So there will be two different solutions for this case. In other words, the phrasing of the question is somewhat deceptive. Usually when the question asks, “Solve the triangle…”, it’s presumed that there is only one solution. In this case, however, there are two different solutions.

These two different solutions appear when using the Law of Sines:

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \gamma}{c}

\displaystyle \frac{\sin 30^\circ}{8} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{1/2}{8} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{5}{8} = \sin \gamma

At this point, the natural inclination of a student is to pop out the calculator and find \sin^{-1} \frac{1}{3}.

SSAcalc4

This is incorrect logic that, as discussed extensively in earlier in this series of posts, there are two angles between 0^\circ and 180^\circ with a sine of 5/8:

\sin^{-1} \frac{5}{8} \qquad \hbox{and} \qquad \pi - \sin^{-1} \frac{5}{8},

or, in degrees,

\gamma \approx 38.68^\circ \qquad \hbox{and} \qquad \gamma \approx 141.32^\circ

So we have two different cases to check. Unlike the previous posts in this series, it’s really, really important that we list both of these cases.

Case 1: \gamma \approx 38.68^\circ. We begin by solving for \beta:

\beta = 180^\circ - \alpha - \gamma \approx 111.32^\circ

Then we can use the Law of Sines to find b. In this case, it’s best to use the pair \alpha - a instead of \gamma - c since the values of \alpha and a are both known exactly.

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \beta}{b}

\displaystyle \frac{\sin 30^\circ}{5} = \displaystyle \frac{\sin 111.32^\circ}{b}

b = \displaystyle \frac{8 \sin 111.32^\circ}{\sin 30^\circ}

b \approx 14.9

This triangle with \gamma \approx 38.68^\circ, \beta \approx 111.32^\circ, and b \approx 14.9 corresponds to the bigger of the two triangles in the above picture, or the rightmost of the two places where the dotted circle intersects the black dotted line.

Case 2: \gamma \approx 141.32^\circ. We again begin by solving for \beta:

\beta = 180^\circ - \alpha - \gamma \approx 8.68^\circ

Unlike yesterday’s example, this is possible. So we have to continue the calculation to find b:

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \beta}{b}

\displaystyle \frac{\sin 30^\circ}{5} = \displaystyle \frac{\sin 8.68^\circ}{b}

b = \displaystyle \frac{8 \sin 8.68^\circ}{\sin 30^\circ}

b \approx 2.4

This second triangle with \gamma \approx 141.32^\circ, \beta \approx 8.68^\circ, and b \approx 2.4 corresponds to the thinner of the two triangles in the above picture, or the leftmost of the two places where the dotted circle intersects the black dotted line.

Inverse functions: Arcsine and SSA (Part 15)

We’ve seen in this series that blinding using the arcsine function on a calculator is insufficient for finding all solutions of an equation like \sin \theta = 0.8. In today’s post, I discuss one of the first places that this becomes practically important: solving the ambiguous case of solving a triangle given two sides and an nonincluded angle.

A note on notation: when solving for the parts of \triangle ABC, a will be the length of the side opposite \angle A, b will be the length of the side opposite \angle B, and c will be the length of the side opposite angle C. Also \alpha will be the measure of \angle A, \beta will be measure of \angle B, and \gamma will be the measure of \angle C. Modern textbooks tend not to use \alpha, \beta, and \gamma for these kinds of problems, for which I have only one response:

philistines

Why does an SSA triangle produce an ambiguous case (unlike the SAS, SSS, or ASA cases)? Here’s a possible problem that has exactly one solution:

Solve \triangle ABC if a = 15, c = 10, and \alpha = 30^\circ.

A student new to the Law of Sines might naively start solving the problem by drawing something like this:

badSSA3

Of course, that’s an inaccurate picture that isn’t drawn to scale. A more accurate picture would look like this:

SSA3

Notice that the red circle intersects the dashed black line at exactly one point. Therefore, we know that there will be exactly one solution for this case. We also note that the circle would have intersected the black dashed line had the dashed line been extended to the left. This will become algebraically clear in the solution below.

Of course, students should not be expected to make a picture this accurately when doing homework. Fortunately, this impossibility naturally falls out of the equation when using the Law of Sines:

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \gamma}{c}

\displaystyle \frac{\sin 30^\circ}{15} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{1/2}{15} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{1}{3} = \sin \gamma

At this point, the natural inclination of a student is to pop out the calculator and find \sin^{-1} \frac{1}{3}.

SSAcalc1

This is incorrect logic that, as discussed extensively in yesterday’s post, there are two angles between 0^\circ and 180^\circ with a sine of 1/3:

\sin^{-1} \frac{1}{3} \qquad \hbox{and} \qquad \pi - \sin^{-1} \frac{1}{3},

or, in degrees,

\gamma \approx 19.47^\circ \qquad \hbox{and} \qquad \gamma \approx 160.53^\circ

So we have two different cases to check.

Case 1: \gamma \approx 19.47^\circ. We begin by solving for \beta:

\beta = 180^\circ - \alpha - \gamma \approx 130.53^\circ

Then we can use the Law of Sines (or, in this case, the Pythagorean Theorem), to find b. In this case, it’s best to use the pair \alpha - a instead of \gamma - c since the values of \alpha and a are both known exactly.

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \beta}{b}

\displaystyle \frac{\sin 30^\circ}{15} = \displaystyle \frac{\sin 130.53^\circ}{b}

b = \displaystyle \frac{15 \sin 130.53^\circ}{\sin 30^\circ}

b \approx 22.8

Case 2: \gamma \approx 130.53^\circ. We again begin by solving for \beta:

\beta = 180^\circ - \alpha - \gamma \approx -10.53^\circ

Oops. That’s clearly impossible. So there is only one possible triangle, and the missing pieces are \gamma \approx 19.47^\circ, \beta \approx 130.53^\circ, and b \approx 22.8. Judging from the above (correctly drawn) picture, these numbers certainly look plausible.

It turns out that Case 2 will always fail in SSA will always fail as long as the side opposite the given angle is longer than the other given side (in this case, a > c). However, I prefer that my students not memorize this rule. Instead, I’d prefer that they list the two possible values of \gamma and then run through the logical consequences, stopping when an impossibility is reached. As we’ll see in tomorrow’s post, it’s perfectly possible for Case 2 to produce a second valid solution with the proper choice for the length of the side opposite the given angle.

Inverse functions: Arcsine and SSA (Part 14)

We’ve seen in this series that blinding using the arcsine function on a calculator is insufficient for finding all solutions of an equation like \sin \theta = 0.8. In today’s post, I discuss one of the first places that this becomes practically important: solving the ambiguous case of solving a triangle given two sides and an nonincluded angle.

A note on notation: when solving for the parts of \triangle ABC, a will be the length of the side opposite \angle A, b will be the length of the side opposite \angle B, and c will be the length of the side opposite angle C. Also \alpha will be the measure of \angle A, \beta will be measure of \angle B, and \gamma will be the measure of \angle C. Modern textbooks tend not to use \alpha, \beta, and \gamma for these kinds of problems, for which I have only one response:

philistines

Why does an SSA triangle produce an ambiguous case (unlike the SAS, SSS, or ASA cases)? Here’s a possible problem that has exactly one solution:

Solve \triangle ABC if a = 15, c = 10, and \alpha = 30^\circ.

A student new to the Law of Sines might naively start solving the problem by drawing something like this:

badSSA3

Of course, that’s an inaccurate picture that isn’t drawn to scale. A more accurate picture would look like this:

SSA3

Notice that the red circle intersects the dashed black line at exactly one point. Therefore, we know that there will be exactly one solution for this case. We also note that the circle would have intersected the black dashed line had the dashed line been extended to the left. This will become algebraically clear in the solution below.

Of course, students should not be expected to make a picture this accurately when doing homework. Fortunately, this impossibility naturally falls out of the equation when using the Law of Sines:

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \gamma}{c}

\displaystyle \frac{\sin 30^\circ}{15} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{1/2}{15} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{1}{3} = \sin \gamma

At this point, the natural inclination of a student is to pop out the calculator and find \sin^{-1} \frac{1}{3}.

SSAcalc1

This is incorrect logic that, as we’ll see tomorrow, nevertheless leads to the correct conclusion. This is incorrect logic because there are two angles between 0^\circ and 180^\circ with a sine of 1/3. There is one solution in the first quadrant (the unique answer specified by arcsine), and there is another answer in the second quadrant — which is between 90^\circ and 180^\circ and hence not a permissible value of arcsine. Let me demonstrate this in three different ways.

First, let’s look at the graph of y = \sin x (where, for convenience, the units of the x-axis are in degrees). This graph intersects the line y = \frac{1}{3} in two different places between 0^\circ and 180^\circ. This does not violate the way that arcsine was defined — arcsine was defined using the restricted domain [-\pi/2,\pi/2], or [-90^\circ, 90^\circ] in degrees.

sinewaveSSA

Second, let’s look at drawing angles in standard position. The angle in the second quadrant is clearly the reflection of the angle in the first quadrant through the y-axis.

standardSSA

Third, let’s use a trigonometric identity to calculate \sin \left( \pi - \sin^{-1} \displaystyle \frac{1}{3} \right):

\sin \left( \pi - \sin^{-1} \displaystyle \frac{1}{3} \right) = \sin \pi \cos \left( \sin^{-1} \displaystyle \frac{1}{3} \right) - \cos \pi \sin \left( \sin^{-1} \displaystyle \frac{1}{3} \right)

=0 \cdot \cos \left( \sin^{-1} \displaystyle \frac{1}{3} \right) + 1 \cdot \sin \left( \sin^{-1} \displaystyle \frac{1}{3} \right)

= \displaystyle \frac{1}{3}

Fourth, and perhaps most convincingly for modern students (to my great frustration), let’s use a calculator:

SSAcalc3

 All this to say, blinding computing \sin^{-1} \frac{1}{3} uses incorrect logic when solving this problem.

green line

Tomorrow, we’ll examine what happens when we try to solve the triangle using these two different solutions for \gamma.

Inverse functions: Arcsine and SSA (Part 13)

We’ve seen in this series that blinding using the arcsine function on a calculator is insufficient for finding all solutions of an equation like \sin \theta = 0.8. In today’s post, I discuss one of the first places that this becomes practically important: solving the ambiguous case of solving a triangle given two sides and an nonincluded angle.

A note on notation: when solving for the parts of \triangle ABC, a will be the length of the side opposite \angle A, b will be the length of the side opposite \angle B, and c will be the length of the side opposite angle C. Also \alpha will be the measure of \angle A, \beta will be measure of \angle B, and \gamma will be the measure of \angle C. Modern textbooks tend not to use \alpha, \beta, and \gamma for these kinds of problems, for which I have only one response:

philistines

Why does an SSA triangle produce an ambiguous case (unlike the SAS, SSS, or ASA cases)? Here’s a possible problem that has exactly one solution:

Solve \triangle ABC if a = 5, c = 10, and \alpha = 30^\circ.

A student new to the Law of Sines might naively start solving the problem by drawing something like this:

badSSA2Of course, that’s an inaccurate picture that isn’t drawn to scale. A more accurate picture would look like this:

SSA2

Notice that the red circle intersects the dashed black line at exactly one point. Therefore, we know that there will be exactly one solution for this case.

Of course, the reason that the dashed circle and line intersect at exactly one point is because a = c \sin \alpha, so that the triangle is a right triangle.

Of course, students should not be expected to make a picture this accurately when doing homework. Fortunately, this impossibility naturally falls out of the equation when using the Law of Sines:

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \gamma}{c}

\displaystyle \frac{\sin 30^\circ}{5} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{1/2}{5} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle 1 = \sin \gamma

90^\circ = \gamma

The jump to the last step is only possible because there’s exactly one angle between 0^\circ and 90^\circ whose sine is equal to 1. In the next couple posts in this series, we’ll see what happens when we get a step where 0 < \sin \gamma < 1.

Anyway, for the problem at hand, from this point forward it’s easy to solve for the remaining pieces. We begin by finding \beta:

\beta = 180^\circ - \alpha - \gamma = 60^\circ

Then we can use the Law of Sines (or, in this case, the Pythagorean Theorem), to find b:

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \beta}{b}

\displaystyle \frac{\sin 30^\circ}{5} = \displaystyle \frac{\sin 60^\circ}{b}

\displaystyle \frac{1/2}{5} = \displaystyle \frac{\sqrt{3}/2}{b}

b = 5\sqrt{3}

green lineIn the next few posts of this series, I’ll consider the other SSA cases — including the case where two solutions are possible.

 

Inverse functions: Arcsine and SSA (Part 12)

We’ve seen in this series that blinding using the arcsine function on a calculator is insufficient for finding all solutions of an equation like \sin \theta = 0.8. In today’s post, I discuss one of the first places that this becomes practically important: solving the ambiguous case of solving a triangle given two sides and an nonincluded angle.

A note on notation: when solving for the parts of \triangle ABC, a will be the length of the side opposite \angle A, b will be the length of the side opposite \angle B, and c will be the length of the side opposite angle C. Also \alpha will be the measure of \angle A, \beta will be measure of \angle B, and \gamma will be the measure of \angle C. Modern textbooks tend not to use \alpha, \beta, and \gamma for these kinds of problems, for which I have only one response:

philistines

Why does an SSA triangle produce an ambiguous case (unlike the SAS, SSS, or ASA cases)? Here’s a possible problem that has no solution:

Solve \triangle ABC if a = 3, c = 10, and \alpha = 30^\circ.

A student new to the Law of Sines might naively start solving the problem by drawing something like this:

badSSAOf course, that’s an inaccurate picture that isn’t drawn to scale. A more accurate picture would look like this:

SSA1The red dashed circle with center B illustrates the dilemma: “side” BC is simply too short to reach the horizontal dashed line to make the vertex C, dangling limply from the vertex B.

Of course, students should not be expected to make a picture this accurately when doing homework. Fortunately, this impossibility naturally falls out of the equation when using the Law of Sines:

\displaystyle \frac{\sin \alpha}{a} = \displaystyle \frac{\sin \gamma}{c}

\displaystyle \frac{\sin 30^\circ}{3} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{1/2}{3} = \displaystyle \frac{\sin \gamma}{10}

\displaystyle \frac{5}{3} = \sin \gamma

Since \sin \gamma must like between 0 and 1 (said another way, \sin^{-1} \frac{5}{3} is undefined), we know that this triangle cannot be solved.

green lineIn the next few posts of this series, I’ll consider the other SSA cases — including the case where two solutions are possible.