NCERT Solutions
Class 11 Maths
Principle of Mathematical Induction

Ex.4.1 Q.16
Prove the following by using the principle of mathematical induction for all n є N:
+
+
+ …………. + [1 ÷ {(3n - 2) (3n + 1)}] =
Let the given statement be P(n), i.e.,
P(n): +
+
+ …………. + [1 ÷ {(3n - 2) (3n + 1)}] =
For n = 1, we have
P (1) = = 1 ÷ (3.1 + 1) =
=
, which is true.
Let P(k) be true for some positive integer k, i.e.,
P(k): +
+
+ …………. + [1 ÷ {(3k - 2) (3k + 1)}] = [k ÷ (3k + 1)] ………... (1)
We shall now prove that P (k + 1) is true.
Consider
+
+
+ …………. + {1 ÷ {(3k - 2) (3k + 1)} + 1 ÷ [{3(k + 1) – 2} {3(k + 1) + 1)}]
= {k ÷ (3k + 1)} + 1 ÷ [{3(k + 1) – 2} {3(k + 1) + 1)}]
[From equation 1]
= [k ÷ (3k + 1)] + [1 ÷ {(3k + 1) (3k + 4)}]
= {1 ÷ (3k + 1)} {k + {1 ÷ (3k + 4)}}
= {1 ÷ (3k + 1)} [{k (3k + 4) + 1} ÷ (3k + 4)]
= {1 ÷ (3k + 1)} {3k2 + 4k + 1} ÷ (3k + 4)
= {1 ÷ (3k + 1)} {3k2 + 3k + k + 1} ÷ (3k + 4)
= {1 ÷ (3k + 1)} {(3k + 1) (k + 1)} ÷ (3k + 4)
= (k + 1) ÷ (3k + 4)
= (k + 1) ÷ {(3(k + 1) + 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.