Question

Over which interval is the graph of f(x) = –x2 + 3x + 8 increasing? (–∞, 1.5) (–∞, 10.25) (1.5, ∞) (10.25, ∞)

Answer 1

The answer is A (-infinity, 1.5)

Answer 2

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.