5sin²x+cosx-1=0
Помогите,пожалуйста!!!

5sin²x + cosx - 1 = 0
5( 1 - cos²x) + cosx - 1 = 0
5 - 5cos²x + cosx - 1 = 0
- 5cos²x + cosx + 4 = 0    /:(-1)
5cos²x  - cosx -  4 = 0
(5cosx + 4)(cosx - 1) = 0

cosx = 1 ==> x = 2pik, k ∈ Z
cosx = - 0,8 ==> x = ± (pi - arccos(0,8)) + 2pik, k ∈ Z