NCERT Solutions
Class 11 Maths
Principle of Mathematical Induction

Ex.4.1 Q.18
Prove the following by using the principle of mathematical induction for all n є N:
1 + 2 + 3 + …………+ [n < (2n + 1)2 ÷ 8]
Let the given statement be P(n), i.e.,
P(n): 1 + 2 + 3 + …………+ [n < (2n + 1)2 ÷ 8]
It can be noted that P(n) is true for n = 1 since
1 < (2.1 + 1)2 ÷ 8 =
Let P(k) be true for some positive integer k, i.e.,
P(k): 1 + 2 + 3 + …………+ [k < (2k + 1)2 ÷ 8] ………. (1)
We shall now prove that P (k + 1) is true.
Consider
1 + 2 + 3 + …………+ k + (k + 1) < {(2k + 1)2 ÷ 8} + (k + 1)
[from equation 1]
1 + 2 + 3 + …………+ k + (k + 1) < {(2k + 1)2 + 8(k + 1)} ÷ 8
1 + 2 + 3 + …………+ k + (k + 1) < {4k2 + 4k + 1 + 8k + 8} ÷ 8
1 + 2 + 3 + …………+ k + (k + 1) < {4k2 + 12k + 9} ÷ 8
1 + 2 + 3 + …………+ k + (k + 1) < (2k + 3)2 ÷ 8
1 + 2 + 3 + …………+ k + (k + 1) < {2(k + 1) + 1}2 ÷ 8
Thus, 1 + 2 + 3 + …………+ k + (k + 1) < (2k + 1)2 ÷ 8 + (k + 1)
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.