Posted by **Tommy** on Friday, July 18, 2008 at 5:52am.

Prove the identity.

6cos^2(x)-1 -12cos^4(x) +8cos^6(x)= cos^3(2x)

Thanks for your help :)

- Trigonometry -
**drwls**, Friday, July 18, 2008 at 6:40am
Use the fact that

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

The left side of your identity is

(2 cos^2x -1)^3

Let's prove that:

= (4 cos^4x -4 cos^2x +1)(2 cos^2x -1)

= 8 cos^6x -12 cos^4x +6 cos^2 -1

Therefore

8 cos^6x -12 cos^4x +6 cos^2 -1

= cos^3(2x)

- Trigonometry -
**Tommy**, Friday, July 18, 2008 at 11:10am
Thank you for your help :)

By the way, i wanna ask which side i should start when i am asked to prove this boring identities and which formulae i should use. There r so many formulaes !!!! You know, i just dun have the "sense of mathematics". My friends can do this sum in 5 mins. I just cant figure how how he can do it.

- Trigonometry -
**drwls**, Friday, July 18, 2008 at 3:00pm
You can start on either side. It all depends upon which side already looks uncomplicated. Try simplifying the other side first. There is no single formula to use. Knowing the double angle formulas and the definitions if tan, cot, sec and csc in therms of sin and cos almost always helps.

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