If f is decreasing and f(20)=10, which of the following must be incorrect? Explain.

a. (f^-1)'(20)=-3
b. (f^-1)'(10)=12

how do i work this problem? i'm kind of confused about inverse functions.

b must be incorrect, because the inverse function f^-1(x) must be 20, not 12, when x = 10

At x = 20, the inverse function could be anything .