You need to expand your cosine identities a bit. Use the one you stated to produce:
This should get you where you are going.
hi there
can anyone get me a little bit further with this question?
prove the identity:
cos 3x = 4cos^3x -3cosx cos 3x can also =cos(2x+x)
so far i have used the formula
cos(a+b) = cosa.cosb-sina.sinb
so this i have equated to
cos(2x+x)=cos2x.cosx-sin2x.sinx
back to the orig equation cos2x.cosx-sin2x.sinx=4cos^3x-3cosx
so thats where i end up and i am now confused as how to proceed next!
any help appreciated.
many thanks in advance sure its nearly there.
hi again,firstly many thanks for the very prompt replies.
i'm sorry to say i not quite getting there which is annoying me.
i know the lhs needs to equal the rhs and that the identities can represent other identities and thats how it is all simplified but i cant seem to convert to get the appropriate identities and the theory is so simple!
my ongoing attempt, is this getting there or not?
cos2x.cosx-sin2x.sinx changes to the following?
2cos^2-1.cosx-sin2x.sinx
2cos^3x-1-sinx.sinx
2cos^3-1-sin^2 2x?
so its all going a bit pete tong as they say
not seeing how this will equate to the rhs i'm afraid and i have honestly been trying for hours now.
it is actually incredibly straight forward isnt it?
i think my problem was i was struggling with the multiplications believe it or not how stupid can you be.
it is a great feeling when it all comes together and you can see where you went wrong ,thankyou again its very clear now.