Elizabeth S. answered • 07/09/20
Electrical Engineering Student
In order to find the interval of increasing and decreasing you have to take the first derivative of f(x). In this case it would be f'(x)= -60x^4+ 420x^3 -720x^3. Using this equatino you would then find your intervals by solving for x. In this case: x= -5 and x=0. Now you have three intervals: -infinity to -5, -5 to 0, and 0 to infinity. In order to determine whether these are intervals of increase or decrease you have to pulg in a number from between these intervals. For this first onne I plug in -6 and I get a negative number so it is an interval of decrease. Next I plug in -4 and get a positive number so it is an interval of decrease. Lastly i plug in 1 and it is negative so it is decreasing.
