По формулам приведения

sin77=sin(90-13)=cos13

cos73=cos(90-17)=sin17

cos13cos17-sin13sin17=cos(13+17)=cos30=корень3/2

Verified answer

sinacosb=0.5(sin(a+b)+sin(a-b))
sina-sinb=2sin((a-b)/2)cos((a+b)/2)
Тогда
sin77cos17-sin13cos73=0.5(sin94+sin60) - 0.5(sin86 +sin(-60))=
=0.5(sin94-sin86+2sin60)=0.5(sin(94-86)/2)cos(94+86)/2)+2sin60)=
=0.5(0+2sin60)=sin60=0.5sqrt3
Удачи.