Answer:
lowest common multiple of 4 and 4 is 4
To verify those for alpha and beta, simply substitute the known values:
sin 30° = cos 60° = 1/2
sin 60° = cos 30° = √3/2
cos 90° = sin 0° = 0
sin -30° = -1/2
sin 120° = √3/2
a.
cos 90° = cos 30° cos 60° - sin 30° sin 60°
0 = √3/2 * 1/2 - 1/2 * √3 / 2 = √3/4 - √3/4 = 0
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b.
sin -30° = sin 30° cos 60° - cos 30° sin 60°
-1/2 = 1/2 * 1/2 - √3/2 * √3/2 = 1/4 - 3/4 = -2/4 = -1/2
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c.
sin 120° = 2 sin 60° cos 60°
√3/2 = 2 * √3/2 * 1/2 = √3/2
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