Prove that:
1. 0
13
5π
cos
13
3π
cos
13
9π
cos
13
π
cos2 =++
2. (sin 3x + sin x) sin x + (cos 3x – cos x) cos x = 0
3. (cos x + cos y)
2
+ (sin x – sin y)
2
= 4 cos2
2
x y
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