Вычислите:
sin77°cos17° - sin13°cos73°;
cos125°cos5° + sin55°cos85°

А почему cos13°cos17°-sin13°sin17°=cos 30?

По формуле косинуса суммы углов.

sin77°cos17°-sin13°cos73°=cos(90°-77°)cos17-sin13°sin(90°-73°)==cos13°cos17°-sin13°sin17°=cos(13°+17°)=cos30°=cos125°cos5°+sin55°cos85°=cos125°cos5°+sin(180°-55°)sin(90°-85°)==cos125°cos5°+sin125°sin5°=cos(125°-5°)=cos120°=