261.Вычислите:
2) cos47°cos17° +sin47°sin17° =cos(47° -cos17°) =cos30° = √3 /2.
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4) (tq70° +tq80°)/(1-tq70°tq80°) =tq(70° +80°) =tq150° =tq(180° -30°) = - tq30° = - √3 /3 .
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262. Упростите :
2) cos30°cos60° - sin30°sin60° =cos(30° +60°) =cos90° = 0.