Square roots and logarithms without a calculator (Part 8)

I’m in the middle of a series of posts concerning the elementary operation of computing a root. This is such an elementary operation because nearly every calculator has a \sqrt{~~} button, and so students today are accustomed to quickly getting an answer without giving much thought to (1) what the answer means or (2) what magic the calculator uses to find roots. I like to show my future secondary teachers a brief history on this topic… partially to deepen their knowledge about what they likely think is a simple concept, but also to give them a little appreciation for their elders.

To begin, let’s again go back to a time before the advent of pocket calculators… say, 1952.

This story doesn’t go back to 1952 but to Boxing Day 2012 (the day after Christmas). For some reason, my daughter — out of the blue — asked me to compute \sqrt[19]{25727} without a calculator. As my daughter adores the ground I walk on — and I want to maintain this state of mind for as long as humanly possible — I had no choice but to comply. So I might as well have been back in 1952.

In the past few posts, I discussed how log tables and slide rules were used by previous generations to perform this calculation. The problem was that all of these tools were in my office and not at home, and hence were not of immediate use.

The good news is that I had a few logarithms memorized:

\log_{10} 2 \approx 0.301, \log_{10} 3 \approx 0.477, \log_{10} 7 = 0.845,

and \ln 10 = 2.3.

I had the first two logs memorized when I was a child; the third I memorized later. As I’ll describe, the first three logarithms can be used with the laws of logarithms to closely approximate the base-10 logarithm of nearly any number. The last logarithm was important in previous generations for using the change-of-base formula from \log_{10} to \ln. It was also prominently mentioned in the chapter “Lucky Numbers” from a favorite book of my childhood, Surely You’re Joking Mr. Feynman, so I had that memorized as well.

I also knew that \ln(1+x) \approx x for x = 0 from the Taylor expansion of \ln(1+x).

green lineTo begin, I first noticed that 25727 \approx 25600, and I knew I could get \log_{10} 25600 since 25600 = 2^8 \times 100. So I started with

\log_{10} 25727 = \log_{10} \left(100 \times 256 \times \displaystyle \frac{257.27}{256} \right)

\log_{10} 25727 \approx \log_{10} 100 + 8 \log_{10} 2 + \log_{10} 1.005

\log_{10} 25727 \approx 2 + 8(0.301) + \displaystyle \frac{\ln 1.005}{\ln 10}

\log_{10} 25727 \approx 4.408 + \displaystyle \frac{0.005}{2.3}

\log_{10} 25727 \approx 4.408 + 0.002

\log_{10} 25727 \approx 4.410

I did all of the above calculations by hand, cutting off after three decimal places (since I had those base-10 logarithms memorized to only three decimal places). Therefore,

\log_{10} 25727^(1/19) = \displaystyle \frac{1}{19} \log_{10} 25727 \approx \displaystyle \frac{4.410}{19} \approx 0.232

So, to complete the calculation, I had to find the value of x so that \log_{10} x = 0.232. This was by far the hardest step, since it could only be done by trial and error. I forget exactly what steps I tried, but here’s a sample:

\log_{10} 2 \approx 0.301. Too big.

\log_{10} 1.5 = \log_{10} \displaystyle \frac{3}{2} = \log_{10} 3 - \log_{10} 2 \approx 0.477 - 0.301 = 0.176. Too small.

\log_{10} 1.6 = \log_{10} \displaystyle \frac{2^4}{10} = 4\log_{10} 2 - \log_{10} 10 \approx 4(0.301) - 1 = 0.203. Too small.

\log_{10} 1.8 = \log_{10} \displaystyle \frac{2 \cdot 3^2}{10} \approx 0.301 + 2(0.477) - 1 = 0.255. Too big.

Eventually, I got to

\log_{10} 1.71 = \log_{10} \displaystyle \frac{3^2 \cdot 19}{100}

\log_{10} 1.71 = 2\log_{10} 3 + \log_{10} 19 - \log_{10} 100

\log_{10} 1.71 \approx 2(0.477) + \displaystyle \frac{\log_{10} 18 + \log_{10} 20}{2} - 2

\log_{10} 1.71 \approx -1.046 + \frac{1}{2} (\log_{10} 2 + 2 \log_{10} 3 + \log_{10} 2 + \log_{10} 10)

\log_{10} 1.71 \approx -1.046 + \frac{1}{2}(2.556)

\log_{10} 1.71 \approx 0.232

So, after a hour or two of arithmetic, I told her my answer: \sqrt[19]{25727} \approx 1.71. You can imagine my sheer delight when we checked my answer with a calculator:


green lineIn Part 9, I’ll discuss my opinion about whether or not these kinds of calculations have any pedagogical value for students learning logarithms.

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