cos125°cos5° + sin55°cos85°

cos(125°) = cos(180° - 55°) = -cos55°

cos(5°) = cos(5°)

sin(55°) = sin(55°)

cos(85°) = sin(90° - 5°) = sin5°

Исходное выражение:

= (-cos55° cos5°) + (sin55° sin5°)

= -cos55° cos5° + sin55° sin5°

= -cos(55°+5°)

= -cos60°

= -1/2

Ответ: -1/2.