Respuesta :

f(x) = -x^2 + 3x + 8
f'(x) = -2x + 3

when f(x) is increasing f'(x)  > 0

-2x + 3 > 0

-2x > -3

x < 3/2 or 1.5

So the required interval is (-infinity,1.5).The first choice.

The answer is A (-infinity, 1.5)