Prove that $2\cosh \left(\frac{1}{2} a\right) \sin \left(\frac {1}{2} \alpha\right) = 1.$

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Suppose a hyperbolic equilateral triangle has side $a$ and angle $\alpha$.Prove that $$2\cosh \left(\frac{1}{2} a\right) \sin \left(\frac {1}{2} \alpha\right) = 1.$$