A quick Trig Identity Question.


by logan233
Tags: identity, trig
logan233
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#1
Jan28-14, 10:45 AM
P: 11
Hopefully this will make sense...

We have the trig. identities shown below:
sin(u)cos(v) = 0.5[sin(u+v) + sin(u-v)]
cos(u)sin(v) = 0.5[sin(u+v) - sin(u-v)]

How are these different? I realize u and v switched between the sine and cosine functions, but what is the difference between u and v? I recognize that there is a difference between taking sine of a number u and sine of a different number v, and same with taking the cosine of a those numbers, I just don't see how we differentiate between u and v. Like say we have...

x(t) = sin(2πt)cos(2π10t) and we choose u = 2πt and v = 2π10t
so that x1(t) = 0.5[sin(2π11t) + sin(2π9t)]
but what if we choose u = 2π10t and v = 2πt
then x2(t) = 0.5[sin(2π11t) - sin(2π9t)], which is different than the original x(t) even though we simply chose u and v to be different?
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jbunniii
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#2
Jan28-14, 12:40 PM
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Quote Quote by logan233 View Post
x(t) = sin(2πt)cos(2π10t) and we choose u = 2πt and v = 2π10t
so that x1(t) = 0.5[sin(2π11t) + sin(2π9t)]
Your mistake is here. The formula is ##0.5[\sin(u+v) + \sin(u-v)]##, so the argument to the second ##\sin## should be ##-2\pi 9 t##, not ##2\pi 9 t##. Then use the fact that ##\sin(-x) = -\sin(x)## to see that the two answers are in fact the same.
logan233
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#3
Jan28-14, 05:14 PM
P: 11
Oh I see now. Thank you.


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