How do I solve?

Question

Assume f(x) = 2x + 8 and g(x) = 5. What is the value of (g o f)(3)?
5
70
18
15

Answer 1

(gof)(x)=g(f(x))

f(x)=2x+8

f(3)=2*3+8

f(3)=6+8

f(3)=14

g(x)=5 for all x

g(f(x))=g(f(3))=g(14)=5 for all x because g(x)=5 for all x, including 14

Answer 2

Hi Adam,

 

You want to find f(g(x)) at 3 or f(g(3)). First thing you have to do is evaluate the function and then substitute the number 3

 

What this means in plain English is that wherever you see x you just put g(x).

 

f(g(x) = 2(g(x)) + 8

 

           = 2*5 + 8

 

           = 18

 

Now you go ahead and evaluate the function for g(x) at x = 3?

 

Wait, isn't g(x) always 5 no matter what x is?

 

That's right.

 

Doesn't that make f(g(x)) 18 no matter what x is? Right again. I hope this helps.