NCERT Solutions
Class 11 Maths
Principle of Mathematical Induction

Ex.4.1 Q.22
Prove the following by using the principle of mathematical induction for all n ∈ N:
32n + 2 – 8n – 9 is divisible by 8.
Let the given statement be P(n), i.e.,
P(n): 32n + 2 – 8n – 9 is divisible by 8.
It can be observed that P(n) is true for n = 1
since 32 × 1 + 2 – 8 × 1 – 9 = 64, which is divisible by 8.
Let P(k) be true for some positive integer k, i.e.,
32k + 2 – 8k – 9 is divisible by 8.
32k + 2 – 8k – 9 = 8m; where m ∈ N ..............(1)
We shall now prove that P (k + 1) is true whenever P(k) is true.
Consider
32(k + 1) + 2 – 8(k + 1) – 9
= 32k + 2 + 2 – 8k - 8 – 9
= 32k + 2 × 32 – 8k – 9
= 32(32k + 2 – 8k – 9 + 8k + 9) – 8k - 17
= 32(32k + 2 – 8k – 9) + 32(8k + 9) – 8k - 17
= 9 × 8m + 9(8k + 9) – 8k - 17
= 9 × 8m + 72k + 81 – 8k - 17
= 9 × 8m + 64k + 64
= 8(9m + 8k + 8)
= 8r, where r = (9m + 8k + 8) is a natural number.
Therefore, 32(k + 1) + 2 – 8(k + 1) – 9 is divisible by 8.
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.