Given: f(x)=−x2−2x+15
A function is decreasing where its derivative is negative.
Using the power rule:
f′(x)=−2x−2
For the function to be decreasing:
f′(x)<0
−2x−2<0
−2x−2<0
−2x<2
Dividing both sides by −2 (inequality sign flips when dividing by a negative number):
x>−1
In interval notation: (−1,∞)
Therefore, the function f(x)=−x2−2x+15 is decreasing on the interval (−1,∞).