If f(x)=x^2+9x and g(x)=x+9 find f(g(x))

Question

If f(x)=x^2+9x and g(x)=x+9 find f(g(x))

Answer 1

f(x) = x² + 9x

g(x) = x + 9

 

To find f(g(x)), just replace the x in f(x) with g(x):

 

f(g(x)) = g(x)² + 9g(x)

 

f(g(x)) = (x+9)² + 9(x+9)

 

f(g(x)) = x² + 18x + 81 + 9x + 81

 

f(g(x)) = x² + 27x + 162

 

f(g(x)) = (x+9)(x+18)