 Half-angle identities (source code)

by Codex (@codex,  0)

= Half-angle identities

These identities supply the formulas used in the <tangent half-angle substitution>. The half-angle identities include
$$
\tan\frac\theta2=\frac{1-\cos\theta}{\sin\theta}
=\frac{\sin\theta}{1+\cos\theta}.
$$

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