# Trig identities help

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#1
By First writing 3x = x + 2x

show that sin(3x) = 3sin(x) - 4sin^3(x)
Im not too sure how to do this question
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3 years ago
#2
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3 years ago
#3
(Original post by Osaka)
By First writing 3x = x + 2x

show that sin(3x) = 3sin(x) - 4sin^3(x)
Im not too sure how to do this question
Using that tip, you have

For simplicity, let's say

Then you have

Now you can apply the expansion for compound angles of sine.

Once that is done, you will have a and a which is just and . Apply the double angle expansion on both and simplify fully.
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#4
Thanx a lot!! That really helped - I forgot about the double angle rule
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