Question

If f(x) = x 2 + 1 and g(x) = 3x + 1, find 2·f(4

Answer 1

Given f(x) = x2+1 and g(x) = 3x+1 we can use the 2nd equation to solve for x in terms of g(x), or, x = (g(x)-1)/3.

Let's put that in the f(x) equation and see what we get: f(x) = ((g(x) - 1)/3)2 + 1. If we let x=4 then this reduces to

((13-1)/3)2+1 = 17 and then multiply by 2 to get 34. You get the same answer.