Explanation:
sin(A − B) = sin A cos B − cos A sin B
sin50°cos 40° - cos50°sin 40°
= sin(50-40)°
=sin10°
cos(A + B) = cos A cos B − sin A sin B
cos75°cos 15° - sin75°sin 15°
= cos(75+15)° = cos90° = 0
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