f(x)=(x−8)4(x−9)5,0≤x≤10,monotonically decreases in

f(x)=(x−8)4(x−9)5,0≤x≤10 ∴ f′(x)=4(x−8)3(x−9)5+5(x−9)4(x−8)4                  =(x−8)3(x−9)4[4(x−9)+5(x−8)]                  =9(x−8)3(x−9)4x−769Sign scheme of f'(x) is as follows:  f′(x)<0 if x∈8,769,i.e., f(x) decreases if x∈8,769.