Решите cos 165° · cos 285° тоже решите sin 75° - вопрос №16528575159192562 от VikaKemer 28.02.2020 09:11

Question

Алгебра: Решите cos 165° · cos 285° тоже решите sin 75° + sin 15°

VikaKemer · Answer

1) cos(285*)*cos(165*) = (1/2)*( cos(285*+165*) + cos(285*-165*) ) == (1/2)*( cos(450*) + cos(120*) ) = (1/2)*( cos(360*+90*) + cos(180*-60*) ) = (1/2)*(cos(90*) - cos(60*) ) = (1/2)*( 0 - (1/2)) = -1/4 = -0,25

2)sin75+sin 15 = 2sin (75+15)/2 * cos (75-15)/2 = 2sin 45*cos30