`sin ^2(x/2) - 2 cos (x/2) + 2 = 0`
` hArr 3 - 2 cos (x/2) - cos (x/2) = 0`
` hArr - [ cos (x/2) - 1][ cos (x/2) + 3] = 0`
` hArr [{:( cos (x/2) - 1 = 0),( cos (x/2) + 3 = 0):}:}( k in bbbZ )`
` hArr [{:( cos (x/2) = 1),( cos (x/2) = - 3):}:}( k in bbbZ )`
` hArr [{:(x/2 = k 2 pi ),(x/2 = arccos ( - 3) + k 2 pi ),(x/2 = - arccos ( - 3) + k 2 pi ):}:}( k in bbbZ ) `
`hArr [{:( x = k 4 pi ),( x = 2 arccos ( - 3) + k 4 pi ),( x = - 2 arccos ( - 3) + k 4 pi ):}:}( k in bbbZ )`
