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Hundredth decimal digit of a huge power of 1 plus root 2

Question

Consider the number (1+2)3000(1+\sqrt2)^{3000}. Find the digit sitting in the 100th place after its decimal point.

Short answer

99. The sum (1+2)3000+(1−2)3000(1+\sqrt2)^{3000}+(1-\sqrt2)^{3000} is an integer, since the odd powers of 2\sqrt2 cancel in the binomial expansions. As 0<(2−1)3000<10−10000<(\sqrt2-1)^{3000}<10^{-1000}, the original number equals an integer minus a tiny positive amount, so its first several hundred decimal digits are all 99.

Adapted from Zhou, A Practical Guide to Quantitative Finance Interviews, sec. 4.2 Combinatorial Analysis, '100th digit', p.70; question and answer rewritten by SelfMadeQuant.com.

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