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A Proof of the Addition Theorem in Trigonometry
Published online by Cambridge University Press: 31 October 2008
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I. Prove sin θ= −sin(− θ), cos θ(− θ)
Let X′OX, Y′OY be rectangular axes, and let OP, OQ, starting from the position OX, describe angles θ, –θ. Then OP, OQ are in every case symmetrically placed with respect to OX. (Illustrate by taking positive and negative values of θ). Hence xP = xQ, yP = –yQ, and the results follow from the definitions of sine and cosine.
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- Copyright © Edinburgh Mathematical Society 1925