1. The reference angle is up/down from either 0° or 180°, so draw the 240° and see what your reference angle is. In this case, it's 180° + 60°, so our reference angle is 60°.
2. Use CAST (or ASTC) to figure out if our answer is going to be positive or negative. In this case it's in the "T" quadrant, so only Tan is positive here.
The exact value is the same as -cos(60), then, which we get from our special triangle. Adj/Hyp is 1/2, so cos240 = -1/2.
2 years ago
Answered By John Luke W
1. The reference angle is up/down from either 0° or 180°, so draw the 240° and see what your reference angle is. In this case, it's 180° + 60°, so our reference angle is 60°.
2. Use CAST (or ASTC) to figure out if our answer is going to be positive or negative. In this case it's in the "T" quadrant, so only Tan is positive here.
The exact value is the same as -cos(60), then, which we get from our special triangle. Adj/Hyp is 1/2, so cos240 = -1/2.