Saturday

September 20, 2014

September 20, 2014

Posted by **Rin** on Tuesday, December 13, 2011 at 6:03pm.

cos3x = 4cos^3x - 3cosx

- trig -
**Reiny**, Tuesday, December 13, 2011 at 6:23pmcos(3x)

= cos(2x + x)

= cos2xcosx - sin2xsinx

= (2cos^2 x -1)cosx - (2sinxcosx)sinx

= 2cos^3 x - cosx - 2sin^2 x (cosx)

= 2 cos^3x - cosx - 2(1 - cos^2 x)(cosx)

= 2 cos^3x - cosx - 2cosx + 2cos^3x

= 4cos^3x - 3cosx

= RS

Nice!

**Answer this Question**

**Related Questions**

trig - By expressing 3x as (2x + x) prove that cos3x = 4cos^3x - 3cosx

trig - how do you verify cos3x=4cos^3x-cosx???

maths... - Differentiate ln [x+√(x^2+1)] with respect to x hence evaluate ...

Trig - Prove (cos^2t+4cos^2+4)/(cost+2)=2sect+1)/sect

trig - sin^4 x = (3-4cos 2x + cos4x)/8 prove on one side please!

Math - Trig - Double Angles - Prove: cos4x = 8cos^4x - 8cos^2x + 1 My Attempt: ...

trig - Write an equation for simple harmonic motion, given that the amplitude ...

Trig - Prove the following functions: (sinx+sin2x)/(1+cosx+cos2x)=tan x (cos3x/...

Trig - Prove (3cosθ-4sinθ)^2+(4cosθ+3sinθ)^2=25

trig - sin2x=cos3x then sinx=?(if 0<x<pi/2)