Im going to assume its (90 - A) so we have:-
1 - sinA sin (90 - A) / cot (90 - A)
Convert to sines and cosines:-
= 1 - (sin A cos A) / cos ( 90 - A) / sin ( 90 - A)
= 1 - (sin A cos A) / sin A / cos A
= 1 - cos^2 A
= sin^2 A which is what is required.
