Hey guys, Im sorry to bothered you but Im kinda desperate and really need your help.

Im preparing for math exam and Ive got a 3 various problem

I know the right answers but Im not able to find out the method. I would really appreciate your help.

1. if cos x = -2/5 than cos 2x = ?? (i know it is -17/25)

or sin x = 3/5 and cos2x= ?? (7/25) but I have no clue how to do it.

2. the number of X (X is from (pi, 2pi) which is valid for sin^2x-sinx=0

(it is 0)

or X is from (0, 2pi) for sin^3x+sinx=0

(it is 1)

3. the last one

the imaginary part of the number

(-1 -i)^8 is (0)

or (1+i)^8 is (16)

(-2 -2i)^8 is (2^12)

(1+i)^32 is (0)

Much appreciate your help and time!