Book: RS Aggarwal - Mathematics
Chapter: 3. Functions
Subject: Maths - Class 11th
Q. No. 15 of Exercise 3F
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15
If 
, where x ≠ –1 and f{f(x)} = x for x ≠ –1 then find the value of k.
Given. 
, x ≠ –1
F(f(x)) = f(
)
= 
= 
Given that f(f(x)) = x
x = 
Dividing both sides by x
1 = 
kx + x + 1 = 
1
– kx – (x+1) = 0
k = 
or k = 
k = 
or k = 
k = 
or k = 
k = x + 1 or k = -1
As value of x is variable we take k = -1.
Therefore, k= -1
