Order of Transformations of a Function
Date: 11/30/2005 at 09:21:42 From: Ozgur Subject: The order in which transformations are performed Could you please tell me in what order I would perform transformations such as -f(x), f(-x), af(x), f(ax), f(x)+a, f(x+a) if two or more were to be applied to f(x)? As an example, if I had f(ax+b) would I do the translation or the stretch first? I've looked in many textbooks and have been unable to find an answer.
Date: 11/30/2005 at 10:19:48 From: Doctor Peterson Subject: Re: The order in which transformations are performed Hi, Ozgur. Good question! I haven't seen this treated well in textbooks, either, but it's an important topic. You can perform transformations in any order you want, in general. But in this case, you are asking in which order to do them in order to transform f(x) into a specific goal, f(ax+b). The order makes a difference in how you get there. What I do is to explicitly write the steps, one at a time. Suppose we first do the horizontal shrink f(x) -> f(ax). If we then apply a horizontal shift (translation) b units to the left, we would be REPLACING x in f(ax) with x+b, and we'd get f(a(x+b)). That is NOT what we are looking for; it's equal to f(ax+ab). So this order of doing those particular transformations is wrong. If instead we first do the shift, changing f(x) to f(x+b), and THEN do the shrink, we replace x in x+b with ax, and get f(ax+b), which is what we want. So that is one answer: Start with: f(x) Shift b units to the left: f(x+b) Shrink horizontally by a factor of a: f(ax+b) But in fact we COULD do the two transformations in the other order, if we change the particular amounts. We can write f(ax+b) as f(a(x+b/a)), factoring out the a, and then do this: Start with: f(x) Shrink horizontally by a factor of a: f(ax) Shift b/a units to the left: f(a(x+b/a)) = f(ax+b) There are a couple things to notice here. First, let's visualize what each pair of transformations does. The first takes the graph of f and moves it left; then shrinks it: | | | | / | / | / | +---+ | +---+ | +-+ | / | / |/ | / |/ |/ +---------- +---------- +---------- The other shrinks first, and then shifts--but not as far, since the shrink reduced the distance it has to go: | | | | / | / | / | +---+ | +-+ | +-+ | / | / |/ | / | / |/ +---------- +---------- +---------- To put it another way, the shrink in the second version also moved the starting point of the graph I drew (by shrinking the empty space), so I had to shift it less to get to the destination graph. A second point to make is that the order of operations determines the order of the transformations. When we wrote the transformed function as f(ax+b), the operations inside the function were done in the order "multiply, then add"; since we are replacing x in each transformation, we ended up doing the last operation first (outside in), replacing x with x+b first, then with a, so we did the shift followed by the shrink. When we wrote it as f(a(x+b/a)), the order of operations says the addition comes first, then the multiplication, so the transformations have to be done in the opposite order. This applies to transformations of x, on the inside of the function. Transformations like a+bf(x) (vertical shifts and stretches) are done in the SAME order as the order of operations. Also, these do not interact with the horizontal transformations, so it doesn't matter which order you do them in; if you had, say, af(bx) you could do the vertical stretch followed by the horizontal shrink, or vice versa. Try it and see! If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Date: 11/30/2005 at 12:27:13 From: Ozgur Subject: Thank you (The order in which transformations are performed) Thank you very much for your answer. Your excellent explanation has cleared up the matter for me.
Date: 09/10/2007 at 22:12:45 From: Elizabeth Subject: Order of Transformations of a Function I read your explanation of transformations order of operations, but I guess I'm still unclear on the order that I have to do them in. For example, if I take the equation y=4 times sq root (2-x), I find that I get the correct graph by doing 1) reflection over y axis 2) horizontal shift of 2 3) vertical stretch of 4 OR 1) vertical stretch 2) reflection 3) horizontal shift. Either way, the horizontal shift has to come after the reflection. It doesn't work if I 1) shift then 2) reflect or stretch 3) reflect or stretch (depending on what I did in step 2). I was always taught to do the horizontal shift first! According to your explanation, as long as what I'm doing yields the correct final equation, I can do whatever order I need to get there. So looking at what didn't work, if I shift first I get sq root (x-2). Then reflect makes sq root (-1)(x-2) or in other words sq root (-x+2) which is the same as sq root (2-x), then finally vertical stretch yields what I want: y=4 times sq root (2-x). But this doesn't graph correctly; the reflection must come before I do the horizontal shift. Why?
Date: 09/10/2007 at 23:37:46 From: Doctor Peterson Subject: Re: Order of Transformations of a Function Hi, Elizabeth. Your question brings up details I didn't emphasize in my original answer. First, I want to point out clearly what each individual transformation looks like, because there are a few points that are easily missed. Here are the transformations mentioned on that page: -f(x) reflection in the x-axis af(x) vertical stretch by factor a f(x)+a vertical shift up by a f(-x) reflection in the y-axis f(ax) horizontal shrink by factor a f(x+a) horizontal shift left by a Note that the first set, the "vertical" transformations, involve changing something OUTSIDE the original function; that is, we do something to the "y" that comes out. The second set, the "horizontal" transformations, involve changing something on the INSIDE of the function. This MUST be seen as REPLACING the x itself with a negative, multiple, or sum. These are the tricky ones. (They are also the ones that tend to act in the opposite direction to what many people expect, shrinking or shifting left rather than stretching or shifting right.) Let's take an example of a reflection in the y-axis for a relatively complicated (compound) function. If f(x) = sqrt(x - 2) then reflection in the y-axis involves NOT replacing x-2 with its opposite, 2-x, but replacing x with its opposite: g(x) = f(-x) = sqrt(-x-2) Try graphing this to see for yourself what happens. A point (x,y) that satisfies y=f(x), reflected in the y-axis to the point (-x,y), will satisfy y=g(x) (using the new value of x). If you graph sqrt(2 - x), you'll see something different. Now, let's break your function down into a series of transformations, starting with the basic square root function: f1(x) = sqrt(x) and heading toward our goal, f(x) = 4 sqrt(2 - x) It doesn't matter how the vertical and horizontal transformations are ordered relative to one another, since each group doesn't interact with the other. Let's do all the horizontal transformations first, since they're the awkward ones. We need to do something with the 2, and something with the -x. As mentioned in the page above, it's most convenient (because of the order of operations) to do the shift FIRST: f1(x) = sqrt(x) f2(x) = f1(x + 2) = sqrt(x + 2) shift left by 2 f3(x) = f2(-x) = sqrt(-x + 2) reflect in y-axis Now we have f3(x) = sqrt(2 - x), and we can apply the vertical transformation: f4(x) = 4 f3(x) = 4 sqrt(2 - x) stretch vertically by 4 So we find that we can shift left by 2, then reflect in the y-axis, and then stretch vertically by 4. And that sequence of transformations works on the graph: * | | * | | * | * * | | | * --> | * --> * | --> * | |* *| |* | *--------- *-+-------- --------+-* *| | |* | | * | --------+-* This is the right graph; for example, f(2) = 4 sqrt(2 - 2) = 0, and f(0) = 4 sqrt(2 - 0) = 4 sqrt(2) = 5.6. Now, this disagrees with what you say: >if I shift first I get sq root (x-2). Then reflect makes >sq root(-1)(x-2) or in other words sq root(-x+2) which is the same >as sq root(2-x), then finally vertical stretch yields what I want: >y=4 times sq root(2-x). But this doesn't graph correctly; the >reflection must come before I do the horizontal shift. Why? You seem to have fallen into the trap I mentioned, negating the entire argument of the sqrt, rather than replacing x with -x, and thinking that you have reflected the graph in the y-axis. Be sure to graph the function you got, and convince yourself that it is not the reflection you think it is. The functions sqrt(x-2) and sqrt(2-x) both have x-intercept x=2, so in fact you have reflected around the line x=2! Similarly, I suspect that when you did the shift after the reflection, you were doing some wrong transformations, and not actually graphing what you got and seeing that it really worked. Please let me know if I'm wrong about that. If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Date: 09/11/2007 at 20:53:18 From: Elizabeth Subject: Thank you (Order of Transformations of a Function) Aha! You are absolutely correct. [of course :) ] I was improperly applying the transformation. Your key phrase "This MUST be seen as REPLACING the x itself with a negative, multiple or sum" cleared it up for me. Thank you for your time and clear explanation!
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