Trigonometric Identities

[MEBC01]

Find cos2β , given that sinβ = −4/5 and 3π/2 <β< 2π

a. - 9/25
b. - 7/25
c. 7/25
d. 9/25
e. None of a – d.

I do not even know where to start with this problem.

 

[Deleted member 4993]

Find cos2β , given that sinβ = −4/5 and 3π/2 <β< 2π

a. - 9/25
b. - 7/25
c. 7/25
d. 9/25
e. None of a – d.

I do not even know where to start with this problem.

Do you know that:

cos(2β) = 1 - 2*sin²(β)

Show us what can you do with that information.

 

[MEBC01]

Do you know that:

cos(2β) = 1 - 2*sin²(β)

Show us what can you do with that information.

I put it into the equation and got my answer.

It was -7/25

Thanks.