Maths Olympiad Prep

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Algebra Difficulty 5.5 AIME, harder Find the answer

Compute the decimal expansion of π\sqrt{\pi}.Yourscorewillbemin(23,k). Your score will be \min (23, k), where kk is the number of consecutive correct digits immediately following the decimal point in your answer.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

For this problem, it is useful to know the following square root algorithm that allows for digit-by-digit extraction of x\sqrt{x}andgivesonedecimalplaceofx and gives one decimal place of \sqrt{x} for each two decimal places of xx. We will illustrate how to extract the second digit after the decimal point of π\sqrt{\pi},knowingthatπ=3.1415, knowing that \pi=3.1415 \cdots and π=1.7\sqrt{\pi}=1.7 \cdots.Let. Let d be the next decimal digit. Then d should be the largest digit such that (1.7+0.01 d)2<πd)^{2}<\pi, which in this case we will treat as (1.7+0.01 d)^{2}<3.1415.Expandingthis,weget. Expanding this, we get 2.89+0.034 d+0.0001 d^{2}< 3.1415, from which we get the value of dtobeapproximately3.14152.890.034=0.25150.034=7 to be approximately \left\lfloor\frac{3.1415-2.89}{0.034}\right\rfloor=\left\lfloor\frac{0.2515}{0.034}\right\rfloor=7, since the 0.0001d20.0001 d^{2} term is negligible. Indeed, 7 is the largest such digit, and so d=7d=7 is the second digit of π$.\sqrt{\pi}\$. Because we are constantly subtracting the square of our extracted answer so far, we can record the difference in a manner similar to long division, which yields a quick method of extracting square roots by hand.

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