Let f(x)=15−∣x−10∣;x∈R. then the set of all values of x, at which the function, g(x)=f(f(x)) is not differentiable, is:
(A){5,10,15}✓ Correct
(B){10}
(C){5,10,15,20}
(D){10,15}
Answer: (A)
Solution
Here f(x)=15−∣x−10∣, with its corner at x=10. Then g(x)=f(f(x))=15−∣5−∣x−10∣∣. The inner ∣x−10∣ already bends the graph at x=10. The outer absolute value bends it wherever 5−∣x−10∣=0, that is ∣x−10∣=5, giving x=5 and x=15. So g fails to be differentiable at x=5,10,15.
JEE Main · 2019 · 09 Apr 2019 · Shift 1 · Single correct option