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NNNLLL54

cosa-sina=-frac{1}{3}\\\1-frac{2}{tga+ctga}=1-frac{2}{frac{sina}{cosa}+frac{cosa}{sina}}=1-frac{2}{frac{sin^2a+cos^2a}{sinacdot cosa}}=\\=1-frac{2cdot sinacdot cosa}{sin^2a+cos^2a}=1-2cdot sinacdot cosa=\\=underbrace {sin^2a+cos^2a}_{1}-2cdot sinacdot cosa=(sina-cosa)^2=(cosa-sina)^2=\\=(-frac{1}{3})^2=frac{1}{9}

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