NCERT Solutions
Class 11 Maths
Sequences and Series

Ex.Misc.Q.7
If f is a function satisfying f (x + y) = f(x). f(y) for all x, y belongs to N, such that
f (1) = 3 and i= = 120, find the value of n.
It is given that, f (x + y) = f (x) × f (y) for all x, y belongs to N ……. (1)
f (1) = 3
Taking x = y = 1 in equation (1), we obtain
f (1 + 1) = f (2) = f (1) × f (1) = 3 × 3 = 9 Similarly,
f (1 + 1 + 1) = f (3) = f (1 + 2) = f (1) × f (2) = 3 × 9 = 27
f (4) = f (1 + 3) = f (1) × f (3) = 3 × 27 = 81
So, f (1), f (2), f (3), …, that is 3, 9, 27, …, forms a G.P. with both the first term and common ratio equal to 3.
It is known that, Sn = a (rn - 1) ÷ (r - 1)
It is given that i=1nf(x) = 120
120 = 3(3n - 1) ÷ (3 - 1)
40 = (3n - 1) ÷ 2
80 = 3n – 1
3n = 81
3n = 34
n = 4
Thus, the value of n is 4.