삼각함수의 덧셈과 곱셈 공식

\(\sin{x} + \sin{y} = 2 \sin\left( \frac{x + y}{2} \right) \cos\left( \frac{x - y}{2} \right)\)

\(\sin{x} - \sin{y} = 2 \cos\left( \frac{x + y}{2} \right) \sin\left( \frac{x - y}{2} \right)\)

\(\cos{x} + \cos{y} = 2 \cos\left( \frac{x + y}{2} \right) \cos\left( \frac{x - y}{2} \right)\)

\(\cos{x} - \cos{y} = -2 \sin\left( \frac{x + y}{2} \right) \sin\left( \frac{x - y}{2} \right)\)

\(\sin{x} \cos{y} = {\sin(x + y) + \sin(x - y) \over 2}\)

\(\cos{x} \sin{y} = {\sin(x + y) - \sin(x - y) \over 2}\)

\(\cos{x} \cos{y} = {\cos(x + y) + \cos(x - y) \over 2}\)

\(\sin{x} \sin{y} = -{\cos(x + y) - \cos(x - y) \over 2}\)

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