Double angle problem

1. Oct 13, 2012

lionely

Evaluate
1-2cos2 25/ 1 -2sin2 65

The attempt at a solution

1-2cos^50= sin^2 50
so sin^2 50 / cos^2 130

I'm stuck and most likely this isn't correct.

Last edited: Oct 13, 2012
2. Oct 13, 2012

HallsofIvy

Staff Emeritus
What you have written makes no sense at all. Surely you don't believe that 2sin(x)= sin(2x)? What trig identities do you know?

Are we to assume that the "25" and "65" are in degrees? Why not just use a calculator to evaluate them?

3. Oct 13, 2012

lionely

Oh I can't use a calculator, that's what it says in the instructions. the identities I know are

cos^2x + sin^2 = 1
1+tan^2x = sec^2 x
cot^2x + 1 = cosec^2x

The addition formulae and the double angle formulae

yes they are in degrees.

4. Oct 13, 2012

lionely

I got it I think. I'm sorry for being stupid, I forgot about the double angle identities for sin^2 and cos^2

I substitued them in and I got 1. Which I think is correct.