sin(a) cos(a) ctg(a) - 1 = sin(a) cos(a) (cos(a) / sin(a)) - 1= cos^2(a) - 1 = -sin^2(a)= -1 + cos^2(a) - 1= cos^2(a) - 2
sin(a) cos(a) ctg(a) - 1 = sin(a) cos(a) (cos(a) / sin(a)) - 1
= cos^2(a) - 1 = -sin^2(a)
= -1 + cos^2(a) - 1
= cos^2(a) - 2