proof of angle sum identities
We will derive the angle sum identities for the various trigonometricfunctions here. We begin by deriving the identity for the sine by meansof a geometric argument and then obtain the remaining identities byalgebraic manipulation.
Theorem 1.
Proof.
Let us make the restrictions and for the time being. Then we may draw atriangle such that and :
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Since the angles of a triangle add up to , we must have, so we have .
We now draw perpendiculars two different ways in order to derive ratios.First, we drop a perpendicular from to :
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Since and are right triangles we have, by definition,
Second, we draw a perpendicular form to . Depending on whether or the point will or will not lie between and , as illustrated below. (There is also the case ,but it is trivial.)
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Either way, and are right triangles, and we have, by definition,
Combining these ratios, we find that
To finish deriving the sum identity, we manipulate the ratios derived abovealgebraically and use the fact that :
To lift the restriction on the range of and , we use the identitiesfor complements and negatives of angles.
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