Question

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

$frac{cosalpha-sin(-alpha) }{1-ctg(-alpha)} =frac{cosalpha-(-sin(alpha))}{1-(-ctg(alpha))} =frac{cosalpha+sinalpha}{1+ctgalpha} =frac{cosalpha+sinalpha}{1+frac{cosalpha}{sina} }=frac{cosalpha+sinalpha}{frac{sinalpha+cosalpha}{sinalpha} }=sinalpha$