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Решите задачу:

$\displaystyle \frac{\cos4а-\cos6а-\cos8а+\cos10а}{\sin^21а\cos1а\cos7а} = \frac{(\cos4а+\cos10а)-(\cos6а+\cos8а)}{\sin^21а\cos1а\cos7а}=\\ \\ \\ = \frac{2\cos7а\cos3а-2\cos7а\cos1а}{\sin^21а\cos1а\cos7а} = \frac{2\cos7а(\cos3а-\cos1а)}{\sin^21а\cos1а\cos7а} =\\\\ \\ = -\frac{8\sin2а\sin1а}{\sin^21а\cos1а}=- \frac{8\sin^21а\cos1а}{\sin^21а\cos1а} =-8$