Find Cos2A: sinA=-1/3, pi<A<3pi/2

[Centurion1]

Homework Statement

Find Cos2A if sinA= -1/3 and pi< (or equal to) A < (or equal to) 3pi/2

Homework Equations

The Attempt at a Solution

Do i use sin^-1 to find when sin equals -1/3

 

[Pengwuino]

Yes, you can do that, that's the simple way. Take note of where your answer occurs as there are 2 points where sin(x) = -1/3. The question tells you the domain of A.

 

[D H]

That's the hard way. The easy way is to use the a identity that expresses cos(2A) in terms of sin(A).

 

[Дьявол]

You should use the identity cos2A = cos²A - sin²A and

cos²A = 1 - sin²A

Regards.