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For which of the following, any number of the form 10𝑛−110𝑛−1 is divisible by 11.For even value of NN=multiple of 11All value of NFor odd value of N

Question

For which of the following, any number of the form 10n10n1 10^n - 10^{n-1} is divisible by 11?

  • For even value of N
  • N = multiple of 11
  • All value of N
  • For odd value of N
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Solution

The number of the form 10^n - 1 is divisible by 11 for all values of N.

Here's why:

Consider the formula for the sum of a geometric series: a + ar + ar^2 + ... + ar^(n-1) = a(1 - r^n) / (1 - r), where a is the first term, r is the common ratio, and n is the number of terms.

In our case, a = 1, r = 10, and n = N. So, our series becomes 1 + 10 + 10^2 + ... + 10^(N-1) = 10^N - 1 / (10 - 1) = 10^N - 1 / 9.

Now, let's consider the divisibility rule for 11: a number is divisible by 11 if and only if the difference between the sum of its digits at odd positions and the sum of its digits at even positions is a multiple of 11.

In our series, the sum of the digits at odd positions is (1 + 0 + 1 + ... + 0 + 1) = N/2 (rounded up to the nearest integer), and the sum of the digits at even positions is (0 + 1 + 0 + ... + 1 + 0) = N/2 (rounded down to the nearest integer). So, their difference is either 0 or 1, depending on whether N is even or odd.

Therefore, 10^N - 1 is divisible by 11 for all values of N.

This problem has been solved

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