Question

cos13cos35+cos77cos55-cos22

Answer 1

cos13*cos35+cos77*cos55-cos22=0

1. cos13=cos(90-77)=sin77
2. cos35=cos(90-55)=sin55
3. cos13*cos35+cos77*cos55-cos22=sin77*sin55+cos77*cos55-cos22=
=cos(77-55)-cos22=cos22-cos22=0

Answer 2

Использованы: формулы приведения, формулы сложения