Prove that: cos(x+y)= cos x cos y - sin x sin y.
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CosX CosY — SinX SinY
= CosX Sin(90°—Y) — SinX Cos(90°—Y)
= Sin[(90°—Y)—X]
= Sin(90°—Y—X)
= Sin[90°—(X+Y)]
= Cos(X+Y)
= CosX Sin(90°—Y) — SinX Cos(90°—Y)
= Sin[(90°—Y)—X]
= Sin(90°—Y—X)
= Sin[90°—(X+Y)]
= Cos(X+Y)
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