Practicing Success

Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Continuity and Differentiability

Question:

The function $f(x)=\cos ^{-1}(\cos x)$, is

Options:

discontinuous at infinitely many-points

everywhere differentiable such that f'(x) = 1

not differentiable at $x=n \pi, n \in Z$ and $f'(x)=1, x \neq n \pi$

not differentiable at $x=n \pi, n \in Z$ and $f'(x)=(-1)^n, x \in(n \pi,(n+1) \pi), n \in Z$

Correct Answer:

not differentiable at $x=n \pi, n \in Z$ and $f'(x)=(-1)^n, x \in(n \pi,(n+1) \pi), n \in Z$

Explanation:

The graph of $f(x)=\cos ^{-1}(\cos x)$ as given in Figure.

It is evident from the curve y = f(x) that f(x) is everywhere continuous but it is not differentiable at $x=n \pi, n \in Z$

Also, $f'(x)=(-1)^n, x \in(n \pi,(n+1) \pi)$, where $n \in Z$

We also observe that f(x) is an even periodic function with period $2 \pi$.