Question

Даю 30 балов
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1-2ctg5alpha/sin10alpha

Answer 1

$\displaystyle\bf\\1-\frac{2Ctg5\alpha }{Sin10\alpha }=1-\frac{\dfrac{2Cos5\alpha }{Sin5\alpha } }{2Sin5\alpha Cos5\alpha } =1-\frac{2Cos5\alpha }{2Sin5\alpha Cos5\alpha \cdot Sin5\alpha } =\\\\\\=1-\frac{1}{Sin^{2} 5\alpha } =\frac{Sin^{2}5\alpha -1 }{Sin^{2} 5\alpha } =\frac{-Cos^{2} 5\alpha }{Sin^{2} 5\alpha } =-Ctg^{2} 5\alpha$