Posted by **Jess** on Saturday, August 6, 2011 at 5:39pm.

How would you solve this trig function:

cos2x=sin^2-2

how do you know what trig identities to use?

- Precalculus -
**Damon**, Saturday, August 6, 2011 at 5:47pm
cos 2x = cos^2 x - sin^2 x identity

so we really have

cos^2 x - sin^2 x = sin^2 x - 2

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

but sin^2 x = 1 - cos^2 x

so we have

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

or

3 cos^2 x = 0

well cos of Pi/2 (90deg) or 3 pi/2 (270 deg) = 0

- Precalculus -
**drwls**, Saturday, August 6, 2011 at 5:51pm
1 - 2sin^2x = sin^2x -2

3 sin^2x = 3

sin^2x = 1

sinx = +1 or -1

x = pi/2 or 3 pi/2

- Precalculus -
**Jess**, Saturday, August 6, 2011 at 6:04pm
oh i get it.... how do you know what identities to use? did you memorize them?

- Precalculus -
**drwls**, Saturday, August 6, 2011 at 11:13pm
The only ones I have memorized are

cos 2x = cos^2x - sin^2 x

sin 2x = 2 sinx cosx,

and cos^x + sin^2 x = 1.

I have an old book with many others that I keep handy.

## Answer This Question

## Related Questions

- Trig Identities - Please help...I'm not understanding trig identities and how to...
- Precalculus/Trig - I can't seem to prove these trig identities and would really ...
- Trig - If sinx = 1/4 Find the exact value of cos2x I used the formula for double...
- trig - Solving Trig Functions: How Do You Solve For Sin 7.5 Using Half-Angle and...
- Trigonometry - Use the trig Identities to find the other 5 trig functions. ...
- trig - I keep trying to find the power reducing formula for sin^4(x), but I can'...
- Trig! - The identities cos(a-b)=cos(a)cos(b)sin(a)sin(b) and sin(a-b)=sin(a)cos(...
- precalculus - how you do you use the power-reducing formula to find the exact ...
- Math - Solve simultaneously for the two equations for x and y 5x(cos 45) + 15y(...
- Trig Identities.. - Solve any way possibly? Thank you! 1-sin^2x/1-cosx

More Related Questions