Consider the following three statements for the function f : (0, ) R defined by f(x) = | e x| - |x - 1| : (I) f is differentiable at all x > 0. (II) f is increasing in (0, 1). (III) f is decreasing in (1, ). Then
- Only (I) is TRUE.
- Only (I) and (III) are TRUE.
- Only (II) and (III) are TRUE.
- All (I), (II) and (III) are TRUE.
Answer
(2) Only (I) and (III) are TRUE.
Solution
For 0 < x < 1: e x < 0, x - 1 < 0
⇒ f(x) = - e x - (1 - x) = x - 1 - e x, f'(x) = 1 - 1x < 0
∴ f is decreasing in (0, 1); (II) is false.
For x > 1: f(x) = e x - x + 1, f'(x) = 1x - 1 < 0
∴ f is decreasing in (1, ); (III) is true.
At x = 1: LHD = 1 - 11 = 0, RHD = 11 - 1 = 0
⇒ f is differentiable at x = 1, and clearly at every other x > 0; (I) is true.
Answer: (2)