Phase Shift in Sine Function
Date: 02/19/99 at 22:37:08 From: Corine Clark Subject: Phase shift in sine function I have a trigonometry test on Monday, and I do not understand phase shift in functions. The problem I am stuck on is y = sin(4x+pi/3). I have figured out that the amplitude is 1, the period is 90 degrees, and the phase shift is -15 degrees. I know how to graph this function without the shift, and I know the graph shifts to the left. What I do not know is how to move the function over correctly. Could you please show me an example of a shifted graph? Thanks for your time. I appreciate it!
Date: 02/20/99 at 15:31:27 From: Doctor Rick Subject: Re: Phase shift in sine function * * * B * * * * * * * * * * * ----*---------------*---------------*--------- *A C* *E * * * * * * * * * D Okay, here is an attempt at drawing a sine curve. I labeled the points A, B, C, D, and E. These are points on the curve that you get when the argument of the sine function (what's inside the parentheses) is 0, pi/2, pi, 3pi/2, and 2pi. Your function is y = sin(4x+pi/3). What will x be at point A? You want 4x+pi/3 = 0 so that y = sin(0) = 0. You can solve for this and you will find x = -pi/12 (which is -15 degrees, but you should leave it as it is). What will x be at point C? You want 4x+pi/3 = pi so that y = sin(pi) = 0 again. You can solve this to get x = pi/6. Since this is twice pi/12, the y axis is 1/3 of the way between A and C. | * * * B * * * | * * * | * * * | * * ----*----+----------*---------------*--------- *| | |* *E *-pi | pi * * * -- | -- * * 12 | 6 * * | * | D Another way of seeing this is to note that when x = 0 you are evaluating sin(pi/3), and pi/3 is 60 degrees, which falls where you see it on the sine curve: 2/3 of the way up the curve toward B. You can work out the other points if you need to. This is a way to be sure you've got it right. You can see now that you had the information you needed: the sine curve is shifted left by pi/12. Perhaps the only thing that was confusing you was that you converted this shift to degrees. - Doctor Rick, 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.