How Cos(A+B) = CosA CosB - SinA SinB ?

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Derivation of Cos(A+B) = CosA CosB - SinA SinB

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How Sin²θ + Cos²θ = 1?

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How   Sin²θ + Cos²θ = 1...?? According to Pythagoras theorem, "the square of hypotenuse(H) is equal to the sum of square of base(B) and perpendicular(P)" i.e.     H² = B² + P² (Dividing both sides by H²) we get,     H²/H² = B²/H² + P²/H²        1   = Sin²θ + Cos²θ                       ....(B/H  = sinθ and P/H = cosθ) Hence, Sin²θ + Cos²θ = 1 How Sin²θ + Cos²θ = 1?
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