1+2*sina*sina-cos2a
=2+(2*sina*sina-1)-cos2a
=2-cos2a-cos2a
=2-2*cos2a
cos(a/2)*cos(a/2)*cos(a/2)*cos(a/2)-sin(a/2)*sin(a/2)*sin(a/2)*sin(a/2)
=(cos(a/2)*cos(a/2)+sin(a/2)*sin(a/2))*(cos(a/2)*cos(a/2)-sin(a/2)*sin(a/2))
=1*cosa
=cosa
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