# How? Which Trig Identityis that?

1. Jun 15, 2013

### k31453

How?? Which Trig Identityis that?

1. The problem statement, all variables and given/known data
[Broken]

2. Relevant equations

3. The attempt at a solution
sin^2 = 1- cos^2 ??

Last edited by a moderator: May 6, 2017
2. Jun 15, 2013

### lurflurf

That looks wrong. It is close to the identity.

$$2\sin^2(x-\pi/4)=1-\sin(2x)$$

3. Jun 15, 2013

### ehild

Yes. one of the relevant equation is sin2θ=1-cos2θ.
The other one is cos(2θ)=cos2θ-sin2θ. From these, you get the identity

sin2θ=.5(1-cos(2θ)). Use it.

ehild

4. Jun 15, 2013

### k31453

but there is no sin..

5. Jun 15, 2013

### ehild

The equation you wrote in the first post is wrong. Correctly, it should be:

2000sin2(500πt)=1000(1-cos(1000πt)).

ehild

6. Jun 15, 2013

### k31453

Awaks.. IT is solution of the Electrical Engineer : priniciples and application Hambley book.

And they made it wrong. I asked the question though i was wrong.