Answer:
No
Step-by-step explanation:
Firstly, observe the function f(x)=-2^x, this function is decreasing and negative (below the x-axis) over its domain.
At the same time, consider the function h(x)=1-x² over the interval 0 ≤ x≤1. this function is decreasing over the said interval. However, it is not negative.
Therefore, a function doesn´t need to be negative over the interval it is decreasing on.
------------------------
Hope it helps...
Have a great day!!