Question

Sin theta equals 5/12 in quadrant, find the exact value of cos 2theta.

I'm not sure how to solve this question, I need some help.

Answer 1

From sinØ = 5/12
make a sketch and use Pythagoras to find the missing side x
x^2 + 5^2 = 12^2

cos 2Ø = 1 - 2 sin^2 Ø
= 1 - 2(5/12)^2
= 1 - 50/144
= 94/144
= 47/72

Answer 2

Did not even have to find the third side, just followed my usual routine.