If f(x) is decreasing for every x > 5, then which choice is not a possibility for f'(x)?

a) f'(x) = x-5

b) f'(x) = 5-x

c) f'(x) = -x^2+10x-25

d) f'(x) = -5

I chose a, am I correct?

Julius
Apr 11, 2018

#1**+1 **

Correct, Julius !!!!

For all x > 5, x - 5 will be positive, thus increasing.....

Also [ don't worry about how I arrived at this ] ....if x - 5 is the derivative, then (1/2)x^2 - 5x + some constant is the parent function....as shown here, no matter the added constant, this function always increases when x > 5

https://www.desmos.com/calculator/mssfiffnb0

CPhill
Apr 11, 2018