If the function f(x) is differentiable
and
f(x) =
ax^3 - 6x ; x <= 1
bx^2 + 4 ; x > 1
then a =
A) 0
B) 1
C) -14
D) -24
E) 26
Yes, the answer is C because as CaptainBlack said, to be differentiable, a function must be continuous, and therefore its derivatives should have values and be continuous. This gives you two equations with two unknowns. You probably had the f(x) values equal to each other at one, but you may have not realized that f'(x) should be continuous at that point as well.