加法定理で &tex{\alpha = \beta = \theta}; と置けば出てくる. #tex{{ \sinh 2 \theta = 2 \sinh \theta \cosh \theta }} #tex{{ \cosh 2 \theta = 2 \cosh^2 \theta -1 = 1+ 2\sinh^2 \theta = \cosh^2 \theta + \sinh^2 \theta }} #tex{{ \tanh 2 \theta = \frac{2 \tanh \theta}{1+ \tanh^2 \theta} }} ここで &tex{\tanh \frac{\theta}{2}=t}; と置くと,次のようにも表せます. #tex{{ \sinh 2 \theta = \frac{2}{1-t^2} }} #tex{{ \cosh 2 \theta = \frac{1+t^2}{1-t^2} }} #tex{{ \tanh 2 \theta = \frac{2t}{1+t^2} }}