Thread: Trig Angles

Q:
1. If tanA=1/3 and pi<A<(3pi/2), calculate cos2A.

2. In what quadrant does the angle 2A lie?

A:
1. I don't need help with this one. Answer: cos2A=4/5 and lies in quadrant 3

2. I need help with this one. Can someone explain to me how I would know which quadrant the angle 2A would be in? How to approach and answer this?

Thanks!

Originally Posted by skeske1234

Q:
1. If tanA=1/3 and pi<A<(3pi/2), calculate cos2A.

2. In what quadrant does the angle 2A lie?

A:
1. I don't need help with this one. Answer: cos2A=4/5 and lies in quadrant 3

Hi skeske1234,

Cos A is negative in the 3rd quadrant and you found Cos 2A = 4/5. Where is Cos A positive? Don't forget your range for angle A.

2. I need help with this one. Can someone explain to me how I would know which quadrant the angle 2A would be in? How to approach and answer this?

Thanks!

Since Tan A = 1/3, that puts A in quadrant I or quadrant III because that's the only 2 quadrants where Tan A is positive.

ArcTan A is about 18.4 degrees in quadrant I and 198.4 in quadrant III. Cos 2A would either be about 36.8 degrees in quadrant I or about 323.2 degrees in quadrant IV which is out of range.