Maths
posted by KK on .
If the minimum value of f(x)=x^2+18x+a is the same as the minimum value of f(f(x)), what is the max value of a?

df/dx = 2x+18
dg/dx = (2f+18)(2x+18)
df/dx=0 when x = 9
dg/dx=0 when f = 9 or x = 9
f = 9 means
x^2+18x+a = 9
x = 9±√(72a)
So, a <= 72