２倍角の公式
- \(\sin2\alpha =2\sin\alpha\cos\alpha\)
- \(\cos2\alpha=\cos^2\alpha-\sin^2\alpha =1-2\sin^2\alpha =2\cos^2\alpha-1\)
- \( \tan2\alpha=\displaystyle\frac{2\tan\alpha}{1-\tan^2\alpha} \)
３倍角の公式
- \(\sin3\alpha =3\sin\alpha-4\sin^3\alpha\)
- \(\cos3\alpha =-3\cos\alpha+4\cos^3\alpha\)
- \(\tan3\alpha =\displaystyle\frac{\tan^3\alpha-3\tan\alpha}{3\tan^2\alpha-1}\)
半角の公式
- \(\sin^2\displaystyle\frac{\alpha}{2} =\frac{1-\cos\alpha}{2}\)
- \(\cos^2\displaystyle\frac{\alpha}{2} =\frac{1+\cos\alpha}{2}\)
- \(\tan^2\displaystyle\frac{\alpha}{2} =\frac{1-\cos\alpha}{1+\cos\alpha}\)
