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# Quick Question - Help!!! watch

1. Hi

looking in syllabus and we need to know how this works:

sin3x = sin(2x+x) = sinx(3-4sin^2(x))

Can anyone please explain this to me??

2. sin3x = sin(2x+x)
= sin2xcosx + cos2xsinx
= 2sinxcosxcosx + (cos²x - sin²x)sinx
= sinx (2cos²x + cos²x - sin²x)
= sinx (3cos²x - sin²x)

then use:
cos²x = 1-sin²x

=sinx (3[1-sin²x]-sin2x)
=sinx (3-4sin²x)
3. (Original post by alwaysingtogeth)
Hi

looking in syllabus and we need to know how this works:

sin3x = sin(2x+x) = sinx(3-4sin^2(x))

Can anyone please explain this to me??

4. Cheers people! Had a question similar in exam today

Prove cos3x = something and I managed to get it right.

Thanks again
5. (Original post by alwaysingtogeth)
Cheers people! Had a question similar in exam today

Prove cos3x = something and I managed to get it right.

Thanks again
no problem. anytime

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