Date: 08/09/2002 at 18:15:16 From: Andy Provan Subject: Inverse functions Hi, I need help on two algebra problems. I found the answer, but I don't know how they got it. Since you have been a big help to me before, I was hoping that you could help me out. Here they are: F(x) = 5x-6, G(x) = x-4 1). (f composed of g)^-1 F(x) = 5x-6, G(x) = x-4 2). G^-1 composed of f^-1 Both of these problems have an answer of x+26/5. Would you please explain at least one of them for me? Thanks again, Andy
Date: 08/09/2002 at 23:11:41 From: Doctor Peterson Subject: Re: Inverse functions Hi, Andy. Your terminology needs a little correction; "f o g" is "f composed WITH g" not "OF g" , which would mean that f is made out of g's. Rather, f and g are "put together," which is what "composed" means. But I've seen enough variation in the use of math words around the world that I may well be wrong. Let's first compose f with g in the first problem: fog(x) = f(g(x)) = f(x-4) = 5(x-4)-6 = 5x - 26 That means we replace "x" in the definition of f with the value of g(x), and then simplify. Now we have to invert this. If y is the value of fog(x), we have y = 5x - 26 Solving for x, y + 26 = 5x (y + 26)/5 = x So the inverse of fog, which takes y back to x, is (fog)^-1(y) = (y + 26)/5 Since the variable in a function definition is just a placeholder, we can replace y with x and get the answer you have (with parentheses where they belong). The second problem is similar, but you will invert each function first, then compose. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.