Sum and Difference Identities

[kaebun]

prove that sin3u=3cos^2usinu-sin^3u
i have no idea where to start

 

[Unco]

sin(3u) = 3cos^2(u)sin(u) - sin^3(u)

You could start by expanding the LHS using the compound angle formula.

Show your progress.

 

[kaebun]

compound angle formula? i don't think i have been taught that

 

[Unco]

sin (A + B) = sin(A)cos(B) + cos(A)sin(B)

 

[kaebun]

oh ok i know that... im still not sure how to use it in this problem though...

 

[Unco]

sin(2u + u) = ...

Spend some time with it; work some things out for yourself.