To solve this type of questions, a practical way is to think of the 1.quadrant situation, that is, model the situation in right triangle trigonometry.
Check the picture.
For the moment let
(we make it positive since we are modeling using a triangle.)
Using the Pythagorean theorem, the length of the side opposite to angle
is found to be 15 units.
We can see that the sine of theta is
.
In the third quadrant, both sine and cosine of an angle are negative, so the actual values of the sine and cosine of theta are, eventually:
, 
.
Recall the double-angle identities (formulas):
, 
.
Using these identities and the ratios found above, we have:
Similarly, applying the double angle formula for the sine we have:
Answer:
Check the picture.
For the moment let
Using the Pythagorean theorem, the length of the side opposite to angle
We can see that the sine of theta is
In the third quadrant, both sine and cosine of an angle are negative, so the actual values of the sine and cosine of theta are, eventually:
Recall the double-angle identities (formulas):
Using these identities and the ratios found above, we have:
Similarly, applying the double angle formula for the sine we have:
Answer: