Ответ: [tex] 1)\; 1-cos^2t=sin^2t\\\\2)\; \; (1-cost)(1+cost)=1-cos^2t=sin^2t\\\\3)\; \; 1- \frac{1}{sin^2t} = \frac{sin^2t-1}{sin^2t} =\frac{-cos^2t}{sin^2t}=-ctg^2t\\\\[/tex]

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