Inverse Functions - Which Statement is True?
Date: 09/20/98 at 21:19:32 From: Faith Smith Subject: Pre-Calculus-Inverse Functions Hi, my question is this: Which statement is true? Prove it. (i) (f composed with g) raised to the (-1) = f raised to the (-1) composed with g raised to the (-1) (ii) (f composed with g) raised to the (-1) = g raised to the (-1) composed with f raised to the (-1) Thank you, Faith
Date: 09/21/98 at 13:27:19 From: Doctor Peterson Subject: Re: Pre-Calculus-Inverse Functions Hi, Faith. I'll just give you a start, so you can have the pleasure of finding the answer yourself. If we have a guess as to the inverse of some function, how can we prove that it really is the inverse? We have to show that it satisfies the definition of inverse. If we say that F is the inverse of f, we have to show that: F o f = I and f o F = I where I'm using "o" for composition and I for the identity function. So for each of your two statements, put the thing on the right for F and the thing under the -1 on the left for f in each of these equations, and see if it is true. For example, for the first, the claim is that: (f^-1 o g^-1) is the inverse of (f o g) so you would want to show that: (f^-1 o g^-1) o (f o g) = I and: (f o g) o (f^-1 o g^-1) = I You have been warned that they aren't both true, so if you get stuck proving one, you should try doing the other. You may want to test each one first with two simple functions (maybe f = 2x and g = x-1) to see if it works. Or maybe you are supposed to know which is right, and should just choose that one and prove it. When you find the answer, you will have shown why, when we solve an equation like 2x - 1 = 3, we "undo" the operations that were done to x (multiplying and then subtracting) in reverse order (adding and then dividing) to get (3+1)/2. - 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.