The question is asking what cos3θcosθ−sin3θsinθ is equivalent to, in the form of either sinnθ or cosnθ, and I'm also supposed to work out n. I've tried rewriting sin3θ as sin(2θ+θ) but I just go down a huge rabbit hole and somehow come out with a quartic.

Help??

Help??

Think about the identity

cos(A+B) = cosAcosB - sinAsinB

https://www.liverpool.ac.uk/~maryrees/homepagemath191/trigid.pdf

cos(A+B) = cosAcosB - sinAsinB

https://www.liverpool.ac.uk/~maryrees/homepagemath191/trigid.pdf

Original post by BobbJo

Think about the identity

cos(A+B) = cosAcosB - sinAsinB

https://www.liverpool.ac.uk/~maryrees/homepagemath191/trigid.pdf

cos(A+B) = cosAcosB - sinAsinB

https://www.liverpool.ac.uk/~maryrees/homepagemath191/trigid.pdf

okay, so I end up here:

Do I try to expand and simplify??

Original post by HollyR3

okay, so I end up here:

Do I try to expand and simplify??

Do I try to expand and simplify??

no

look at cos3θcosθ − sin3θsinθ

look at cos(A+B) = cosAcosB - sinAsinB

Original post by BobbJo

no

look at cos3θcosθ − sin3θsinθ

look at cos(A+B) = cosAcosB - sinAsinB

look at cos3θcosθ − sin3θsinθ

look at cos(A+B) = cosAcosB - sinAsinB

oohhhhhhh I'm with you now, thanks!

Original post by Capossiali

oohhhhhhh I'm with you now, thanks!

But u didnt give the answer, im still stuck

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