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misal
A + B = 5a
A - B = a
maka A = 3a , B = 2a
cos (A + B) + cos (A - B) = 2cosAcosB
cos 5a + cos a = 2cos3acos2a
cos a + 2cos3a + cos5a = 4cos3acos²a
2cos3a + cos5a + cos a = 4cos3acos²a
2cos3a + 2cos3acos2a = 4cos3acos²a
2cos3a (1+ cos2a) = 4cos3acos²a
2cos3a (1 + cos²a - sin²a) = 4cos3acos²a
2cos3a (1 + cos²a - (1-cos²a) = 4cos3acos²a
2cos3a (1 + cos²a - 1 + cos²a) = 4cos3acos²a
2cos3a (2cos²a) = 4cos3acos²a
4cos3acos²a = 4cos3acos²a
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