1.Доказать, что: Cos24 + cos48 - cos84 - cos12=1/2

2.а)
3cos(25°)cos(15°)cos(35°)

б)4sin(25°)cos(15°)sin(5°)

Cos24 + cos48 - cos84 - cos12=1/2sin

cos(24)+cos(48) = 2cos(36)cos(12)

cos(84)+cos(12) = 2cos(48)cos(36)

cos(24)+cos(48)-(cos(84)+cos(12) ) = 2cos(36)(cos(12)-cos(48)) = -4cos(36)sin(30)sin(-18) = 2cos(36)sin(18) = 2cos(36)sin(36)sin(18)/sin(36) = sin(72)sin(18)/sin(36) = sin(72)cos(72)/sin(36) = (1/2)*sin(144)/sin(36) = 1/2

Объяснение:

я не знаю откуда я знаю