NCERT Solutions
Class 11 Maths
Principle of Mathematical Induction

Ex.4.1 Q.23
Prove the following by using the principle of mathematical induction for all n ∈ N:
41n – 14n is a multiple of 27.
Let the given statement be P(n), i.e.,
P(n): 41n – 14n is a multiple of 27.
It can be observed that P(n) is true for n = 1
since 411 – 141 = 27, which is a multiple of 27.
Let P(k) be true for some positive integer k, i.e.,
41k – 14k is a multiple of 27
So, 41k – 14k = 27m, where m ∈ N ............. (1)
We shall now prove that P (k + 1) is true whenever P(k) is true.
Consider
41k+1 – 14k+1
= 41k × 41 – 14k × 14
= 41(41k – 14k + 14k) - 14k + 14
= 41(41k – 14k) + 41 × 14k - 14k + 14
= 41 × 27m + 14k (41 – 14)
= 41 × 27m + 27 × 14k
= 27(41m + 14k)
= 27 × r, where r = (41m + 14k) is a natural number.
Therefore, 41k+1 – 14k+1 is a multiple of 27
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.