It's so big

Algebra Level 3

100 + 10 0 2 + 10 0 3 + + 10 0 2011 \large 100 + 100^2 + 100^3 + \cdots + 100^{2011}

Find the number of digits of the sum above.


The answer is 4023.

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2 solutions

S = 100 + 10 0 2 + 10 0 3 + . . . + 10 0 2011 = 1 0 2 + 1 0 4 + 1 0 6 + . . . + 2 4018 + 2 4020 + 1 0 4022 = 1 0 4022 + 1 0 4020 + 2 4018 + . . . + 2 6 + 1 0 4 + 1 0 2 = 1 00...000 4022 0’s + 1 00...000 4020 0’s + 1 00...000 4018 0’s + . . . + 1 000000 6 0’s + 1 0000 4 0’s + 1 00 2 0’s = 101010...01010100 4023 digits \begin{aligned} S & = 100 + 100^2 + 100^3 + ...+ 100^{2011} \\ & = 10^2 + 10^4 + 10^6+...+2^{4018}+2^{4020}+10^{4022} \\ & = 10^{4022} + 10^{4020} + 2^{4018} + ... + 2^6 + 10^4 + 10^2 \\ & = 1\underbrace{00...000}_{\text{4022 0's}} + 1\underbrace{00...000}_{\text{4020 0's}} + 1\underbrace{00...000}_{\text{4018 0's}} + ... + 1\underbrace{000000}_{\text{6 0's}} + 1\underbrace{0000}_{\text{4 0's}} + 1\underbrace{00}_{\text{2 0's}} \\ & = \underbrace{101010...01010100}_{\boxed{4023} \text{ digits}} \end{aligned}

Shyam Upadhyay
Sep 5, 2016

Here number of digit only depend on last digit 100^(2011) ie 10^(4022) . Take log then number of digit will be log 10^(4022) +1 =4023

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