Composition Functions with Added x Value
Date: 05/13/2001 at 13:21:44 From: Eileen Gonzalez Subject: Composition functions with added "x " value Hello, I would really appreciate some direction on this question. It throws me off because it adds another value for x. How to plug this in? f(x) = 5x and g(x) = 2x^2-x+1 The question reads: If x = 1, evaluate g(f(f(x))) I'm confused with this added value of x = 1. What I did to solve was: f(5(5*1)) f(25*1) 2(25*1)^2-25*1+1 = 2*625-25+1 = 1250-25+1 = 1226 Is this right? I feel as if I'm getting off track with x = 1, but can't be sure. Many thanks, Eileen
Date: 05/13/2001 at 23:11:09 From: Doctor Peterson Subject: Re: Composition functions with added "x " value Hi, Eileen. I don't know why you started with f(5(5*1)); that would be f(f(f(1))). Ah - you must have meant g(5(5*1)), which is what you want to do. And you did it right. There are two ways to do this kind of problem. You can either start by determining the composition g(f(f(x))) in general, and then plugging in the given value of x; or you can find g(f(f(1))) specifically. You did the latter. Let's do it the longer way and think about what this all means. f(x) = 5x g(x) = 2x^2 - x + 1 f(f(x)) = f(5x) = 5(5x) = 25x g(f(f(x))) = g(25x) = 2(25x)^2 - (25x) + 1 = 1250x^2 - 25x + 1 Now we can plug in x = 1, and we get g(f(f(1))) = 1250(1)^2 - 25(1) + 1 = 1226 This separates out the two things that are happening: composition of functions, and evaluation for a specific value. What are we doing? The composition of functions can be seen as hooking up three machines together: x --> f --> f --> g --> ? Your method (which is the quicker way) just feeds 1 into the pipeline, and collects the 1226 that drips out the other end. My method (which would be better if you had to do the work for different values of x) analyzes the machine and replaces it with a single machine that does the same thing: x --> ffg --> ? Then when I'd finished all the plumbing, I put a 1 into the input and got my 1226 out the end. A lot of work for one drop of output, but it helped me understand the process better, and that's what composition of functions is really about: this concept that you have not just a single value to think about, but that the whole pipeline is really a whole new function that can be thought of just the same way you think of any other function. Rather than think of a series of things that happen to one value (your approach), we think of a single bigger thing that can happen to any value (mine). The fact that they give the same answer is what we're after. I hope that helps. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Date: 05/14/2001 at 14:49:08 From: Eileen JQ Gonzalez Subject: Re: Composition functions with added "x " value Dr. Peterson, You just made my day! Thank you so much for your prompt response! You've definitely cleared a few things up for me. Eileen.
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