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

where do i even start? :o

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- Dec 19th 2007, 07:34 AMohhhhprecalc problem help
show that cos 3(theta) = 4cos^3(theta)-3cos(theta)

where do i even start? :o - Dec 19th 2007, 07:42 AMcolby2152
Is this want you would like to prove?

- Dec 19th 2007, 08:00 AMohhhh
yess... thanks for making it the way it should! :)

- Dec 19th 2007, 08:09 AMJaneBennet
- Dec 19th 2007, 08:29 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? - Dec 19th 2007, 08:50 AMKrizalid
- Dec 19th 2007, 05:26 PMohhhh
thanks a lot for your help guys. got it down! :)

- Dec 19th 2007, 05:31 PMbadgerigarQuote:

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

Quote:

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