Write the expression as the sine or cosine of an angle.
cos 7y cos 3y sin 7y sin 3y
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.
Hint: sin(A + B) = sin A cos B = cos A sin B
cos(A B) = cos A cos B i sin A sin B

Question 2 (essay worth 10 points) (03.02 mc) in δabc shown below, point a is at (0, 0), point b is at (x2, 0), point c is at (x1, y1), point d is at x sub 1 over 2, y sub 1 over 2, and point e is at the quantity of x sub 1 plus x sub 2 over 2, y sub 1 over 2: triangle abc is shown. point d lies on segment ac and point e lies on segment bc. a segment is drawn between points d and e. point a is at the origin. prove that segment de is parallel to segment ab.