show that cos 3(theta) = 4cos^3(theta)-3cos(theta)

where do i even start? :o

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- December 19th 2007, 06:34 AMohhhhprecalc problem help
show that cos 3(theta) = 4cos^3(theta)-3cos(theta)

where do i even start? :o - December 19th 2007, 06:42 AMcolby2152
Is this want you would like to prove?

- December 19th 2007, 07:00 AMohhhh
yess... thanks for making it the way it should! :)

- December 19th 2007, 07:09 AMJaneBennet
- December 19th 2007, 07:29 AMohhhh
ok, so let me see.

cos (2a+a) = cos2(a)*cos(a) - sin2(a)*sin(a)

= cos^2a-sin^2a - 2sin(a)*cos(a)*sin(a)

= cos^2a-sin^2a - 2sin^2(a)*cos(a)

...is this right so far? - December 19th 2007, 07:50 AMKrizalid
- December 19th 2007, 04:26 PMohhhh
thanks a lot for your help guys. got it down! :)

- December 19th 2007, 04:31 PMbadgerigarQuote:

cos (2a+a) = cos2(a)*cos(a) - sin2(a)*sin(a)

Quote:

= cos^2a-sin^2a - 2sin(a)*cos(a)*sin(a)