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Решите задачу:

$\frac{ctg^2(a + \frac{pi}{2}) * cos^2(a - \frac{pi}{2} ))} {ctg^2(a - \frac{pi}{2} ) - cos^2(a + \frac{pi}{2} )}$

$\frac{tg^2(a ) * sin^2(a))} {tg^2(a) - sin^2(a)}$

$\frac{ \frac{sin^2(a)}{cos^2(a)} * sin^2(a))} { \frac{sin^2(a)}{cos^2(a)} - sin^2(a)}$

$\frac{sin^4(a)} { sin^2(a) - cos^2(a)sin^2(a)}$

$\frac{sin^4(a)} {sin^2(a)( 1 - cos^2(a))}$

$\frac{sin^2(a)} {1 - cos^2(a)}$

$\frac{sin^2(a)} {sin^2(a)} = 1$