Functions 3 Question 2

2. If f(x)=sinx+cosx,g(x)=x21, then gf(x) is invertible in the domain

(2004, 1M)

(a) 0,π2

(b) π4,π4

(c) π2,π2

(d) [0,π]

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Answer:

Correct Answer: 2. (d)

Solution:

  1. We have, f(x)=|x1|x=(x1)x, if 0<x1 x1x, if x>1

1x1, if 0<x111x, if x>1

Now, let us draw the graph of y=f(x)

Note that when x0, then f(x), when x=1, then f(x)=0, and when x, then f(x)1

Clearly, f(x) is not injective because if f(x)<1, then f is many one, as shown in figure.

Also, f(x) is not surjective because range of f(x) is [0,[ and but in problem co-domain is (0,), which is wrong.

f(x) is neither injective nor surjective



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