Answer:
No. The sign of a function is not related with the fact that it is growing of decreasing.
If the function in the decreasing in the interval (a,b) it means that f(b) < f(a), but yet both f(b) and f(a) may be positive.
For example imagin the function y = - x. It is a straight line and it is decreasing for any interval that you take; this is (-∞, ∞).
If you take the interval (- ∞, 0) the function is decreasing but the values are positive. If you take the interval (0, ∞) the function is decreasing and the values are negative.
Explanation: