Define \(g\) by \(g(x)=5x-4\). If \(g(x)=f^{-1}(x)-3\) and \(f^{-1}(x)\) is the inverse of the function \(f(x)=ax+b\), find \(5a+5b\).
f(x) = ax + b
y = ax + b get x by itself
y - b = ax
[y -b ] / a = x "swap" x and y
[x - b ] / a = y = f-1(x)
So
f-1(x) - 3 = g (x)
[ x - b ] / a - 3 = 5x - 4 add 3 to both sides
[ x - b ] / a = 5x - 1 simplify
(1/a)x - b/a = 5x - 1
Equating terms
(1/a) = 5 ⇒ 5a = 1 ⇒ a = 1/5
And
b/a = 1
b /(1/5) = 1
b = 1 * (1/5)
b = 1/5
So
5a + 5b =
5(1/5) + 5 (1/5) =
1 + 1 =
2