f(x)=-5(x-4)^2+80
f(x)=-5(x²-8x+16)+80
f(x)=-5x²+40x-80+80
f(x)=-5x²+40x
Replace f(x) to y, now switch the location of the variables x and y. Solve for new y.
y=-5x²+40x
x=-5y²+40y
0=-5y²+40y-x
Use quadratic formula to find the value of y.
y=-40+-√(40^2 -4(-5)(-x)/2(-5)
f^-1(x)= (-40+-√1600-20x)/-10 <--- Not too sure about this one :/
Hope I helped :)
