Корень из 1-1/5 cos x равен sin x

√ (1 - (1/5) ·соsx) = sinx.

1 - (1/5) ·cosx=sin²x,

1 - (1/5) ·cosx=1-cos²x,

(1/5) cosx=cos²x,

cos²x - (1/5) cosx=0,

5cos²x-cosx=0,

cosx (cosx-1) = 0,

cosx=0 или cosx=1,

x1=π/2 + πn,

x2=πn, n∈Z.