# Thread: trig sine angle

1. ## trig sine angle

i need help on this question:
Find the exact value of sin 67.5 , i know that 2a is 135. and the terminal angle is 45 degree.

I also need help on this, Express Pie / 2 in terms of cos A.

2. Originally Posted by lickman
i need help on this question:
Find the exact value of sin 67.5 , i know that 2a is 135. and the terminal angle is 45 degree.
[snip]
$\cos(2 \times 67.5^o) = \cos (135^o) = - \cos(45^o)$.

But from the double angle formula, $\cos(2 \times 67.5^o) = 1 - 2 \sin^2 (67.5^o)$.

Therefore ....

3. arent they asking for sin?? the exact value of sin 67.5, so wouldnt u use
Sin(2 x 67.5) = 2(1/square Root 2) (1/square root 2)

4. Originally Posted by lickman
arent they asking for sin?? the exact value of sin 67.5, so wouldnt u use
Sin(2 x 67.5) = 2(1/square Root 2) (1/square root 2)
And how is that going to give you the value of sin(67.5), pray tell?

Solve $-\sqrt{2} = 1 - 2 \sin^2 (67.5^o)$ as I suggested.