sin (Π/2 - theta) = sin(Π/2) cos(theta) - cos(Π/2) sin (theta)
and as we know sin(Π/2) =1 , cos(Π/2)=0
so sin(Π/2 - theta) = 1 cos(theta) - 0 sin(theta)= cos(theta)
2nd cos(180+ theta) = cos(180)cos(theta) - sin(180) sin(theta)
,,, so , cos(180+theta) = -cos(theta)
