NCERT Solutions
Class 11 Maths
Principle of Mathematical Induction

Ex.4.1 Q.7
Prove the following by using the principle of mathematical induction for all n є N:
1.3 + 3.5 + 5.7 + …………+ (2n - 1) (2n + 1) = n (4n2 + 6n - 1) ÷ 3
Let the given statement be P(n), i.e.,
P(n): 1.3 + 3.5 + 5.7 + …………+ (2n - 1) (2n + 1) = n (4n2 + 6n - 1) ÷ 3
For n = 1, we have
P (1): 1.3 = 3 = 1(4.12 + 6.1 - 1) ÷ 3 = (4 + 6 - 1) ÷ 3 = 9 ÷ 3 = 3, which is true.
Let P(k) be true for some positive integer k, i.e.,
1.3 + 3.5 + 5.7 + …………+ (2k - 1) (2k + 1) = k (4k2 + 6k - 1) ÷ 3 …………… (1)
We shall now prove that P (k + 1) is true.
Consider
(1.3 + 3.5 + 5.7 + ... + (2k – 1) (2k + 1) + {2(k + 1) – 1} {2(k + 1) + 1}
= {k (4k2 + 6k - 1) ÷ 3} + {2k + 2 – 1} {2k + 2 + 1}
[From equation 1]
= {k (4k2 + 6k - 1) ÷ 3} + (2k + 1) (2k + 3)
= {k (4k2 + 6k - 1) ÷ 3} + (4k2 + 8k + 3)
= {k (4k2 + 6k - 1) + 3(4k2 + 8k + 3)} ÷ 3
= (4k3 + 6k2 - k + 12k2 + 24k + 9)} ÷ 3
= (4k3 + 18k2 + 23k + 9)} ÷ 3
= (4k3 + 14k2 + 9k + 4k2 + 14k + 9) ÷ 3
= {k (4k2 + 14k + 9) + 1(4k2 + 14k + 9)} ÷ 3
= {(k + 1) (4k2 + 14k + 9)} ÷ 3
= {(k + 1) (4k2 + 8k + 4 + 6k + 6 - 1)} ÷ 3
= [(k + 1) {4(k2 + 2k + 1) + 6(k + 1) - 1}] ÷ 3
= [(k + 1) {4(k + 1)2 + 6(k + 1) - 1}] ÷ 3
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.