Третий номер решите (cos20p/3-sin17p/6)*sin(-27.5p): cos(-31p/4)

Будем делать по частям:
1) Cos20π/3=Cos( 7π - π/3) = -Сosπ/3= -1/2
2) Sin17π/6 = Sin(3π - π/6)= Sin π/6 = 1/2
3) Sin(-27,5π) =-Sin27,5π= -Sin(27π + 0,5π)=-1·(-1) = 1
4)  Cos(-31π/4) = Cos31π/4 = Cos(8π - π/4)= Сos π/4= √2/2
Теперь сам пример:
(-1/2 - 1/2)·1 : √2/2 = -1:√2/2 = 2/√2 = √2