Question

Cosa- sin(-a) /1-ctg(-a)
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Answer 1

$\frac{cos\alpha-sin(-\alpha) }{1-ctg(-\alpha)} =\frac{cos\alpha-(-sin(\alpha))}{1-(-ctg(\alpha))} =\frac{cos\alpha+sin\alpha}{1+ctg\alpha} =\frac{cos\alpha+sin\alpha}{1+\frac{cos\alpha}{sina} }=\frac{cos\alpha+sin\alpha}{\frac{sin\alpha+cos\alpha}{sin\alpha} }=sin\alpha$