2.
(√(1-(1/5)*cosx))²=(sin(x))²
1-(cos(x)/5=sin²(x)
1-(cos(x)/5=1-cos²(x)
-cos(x)/5=-cos²(x) |×(-5)
cos(x)=5*cos²(x)
5*cos²(x)-cos(x)=0
cos(x)*(5*cos(x)-1)=0
cos(x)=0
x₁=π/2+πn
5*cos(x)=1 |÷5
cos(x)=1/5
x₂=arccos(1/5)+2πn.
Ответ: x₁=π/2+πn x₂=arccos(1/5)+2πn.