Question

Find cos2A when tanA = 1/3 and pi 13 years ago

Answer 1

What does pi<A<3pi/2 mean? Are these point on a graph?

Answer 2

\[\tan A=\frac{\sin A}{\cos A}=\frac{1}{3}\]

Answer 3

It means that the angle is in the third quadrant because it is between pi and 3pi/2

Answer 4

Ooh

Answer 5

So
\[\cos 2A=2\cos ^2A-1=2(\frac{-3}{\sqrt{10}})^2-1=2(\frac{9}{10})-1=\frac{8}{10}=\frac{4}{5}\]

Answer 6

Why did cos2A turn to 2cos^2A? or like, how?

Answer 7

Because of the identity:
\[\cos 2A=2\cos ^2A-1\]

Answer 8

oh, right lol thank you (:

Answer 9

yw