NCERT Solutions
Class 11 Maths
Principle of Mathematical Induction

Ex.4.1 Q.5
Prove the following by using the principle of mathematical induction for all n є N:
1.3 + 2.32 + 3.33 + n3n = {(2n - 1)3n-1 + 3} ÷ 4
Let the given statement be P(n), i.e.,
P(n): 1.3 + 2.32 + 3.33 + n3n = {(2n - 1)3n-1 + 3} ÷ 4
For n = 1, we have
P (1): 1.3 = 3 = {(2 × 1 - 1)31-1 + 3} ÷ 4 = (32 + 3) ÷ 4 = 12 ÷ 4 = 3, which is true.
Let P(k) be true for some positive integer k, i.e.,
1.3 + 2.32 + 3.33 + k3k = {(2k - 1)3k-1 + 3} ÷ 4 ………… (1)
We shall now prove that P (k + 1) is true.
Consider
1.3 + 2.32 + 3.33 + ... + k.3k + (k + 1).3k+1
= (1.3 + 2.32 + 3.33 + ... + k.3k) + (k + 1).3k+1
= [{(2k - 1)3k-1 + 3} ÷ 4] + (k + 1).3k+1
[Using equation 1]
= {(2k - 1)3k-1 + 3 + 4(k + 1).3k+1} ÷ 4
= [3k+1{2k - 1 + 4(k + 1)} + 3] ÷ 4
= {3k+1(6k + 3) + 3} ÷ 4
= {3k+1 × 3(2k + 1) + 3} ÷ 4
= {3k+1+ 1(2k + 1) + 3} ÷ 4
= [3(k+1) + 1 {2(k + 1) – 1} + 3] ÷ 4
= [2(k + 1) – 1}3(k+1) + 1 + 3] ÷ 4
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.