Apply half‑angle formulas (#476)

Apply half‑angle formulas (#476)

Use sin⁡2A2=1−cos⁡A2\sin^2\frac{A}{2} = \frac{1-\cos A}{2}, cos⁡2A2=1+cos⁡A2\cos^2\frac{A}{2} = \frac{1+\cos A}{2}.