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**Soroban** Hello, Googl!

You should be familiar with these identities:

. . $\displaystyle \cos\theta \:=\:1-2\sin^2\frac{\theta}{2}$

. . $\displaystyle \sin\theta \:=\:2\sin\frac{\theta}{2}\cos\frac{\theta}{2}$

$\displaystyle \text{The right side is: }\:1 - \left(2\sin\frac{\theta}{2}\cos\frac{\theta}{2}\ri ght)\left(\dfrac{\sin\frac{\theta}{2}}{\cos\frac{\ theta}{2}}\right) $

. . . . . . . . . . . $\displaystyle =\;1 - 2\sin^2\frac{\theta}{2}$

. . . . . . . . . . . $\displaystyle =\;\cos\theta $