NCERT Solutions
Class 11 Maths
Principle of Mathematical Induction

Ex.4.1 Q.20
Prove the following by using the principle of mathematical induction for all n ∈ N:
102n – 1 + 1 is divisible by 11.
Let the given statement be P(n), i.e.,
P(n): 102n – 1 + 1 is divisible by 11.
It can be observed that P(n) is true for n = 1
since P (1) = 102.1 – 1 + 1 = 11, which is divisible by 11.
Let P(k) be true for some positive integer k,
i.e., 102k – 1 + 1 is divisible by 11.
So, 102k – 1 + 1 = 11m, where m ∈ N ..............(1)
We shall now prove that P (k + 1) is true whenever P(k) is true.
Consider
= 102(k + 1) - 1 + 1
= 102k + 2 - 1 + 1
= 102 (102k + 2 - 1 + 1 - 1) + 1
= 102 (102k + 2 - 1 + 1) - 102 + 1
= 102. 11m – 100 + 1
= 100 × 11m – 100 + 1
= 100 × 11m – 99
= 11(100m – 9)
= 11r, where r = (100m - 9) is some natural number.
Therefore, 102(k + 1) - 1 + 1is divisible by 11.
Thus, P (k + 1) is true whenever P(k) is true.
Hence, by the principle of mathematical induction, statement P(n) is true for all natural numbers i.e., N.