First of all thanks for your reply. Here is another question about trigonometric equation.
cos^(x) + cos (x)= sin^x on 0 degree < x< 360
cos^(x) + cos (x)= 1-cos^(x)
2cos ^(x) + cos (x)-1= 0
(2cos(x) -1)(cos(x) + 1= 0
cos(x)=1/2 , cos x = -1
so complete solution is
60 degree,180 degree, 300 degree.
AS from reference angle , I know that cos 1/2 = 60 degree .But how did the book get the complete solution like 180 and 300 degree .(from where came the 180 and 300 degree)