вычислить cos125°cos5°+sin55°cos85°

Question

вычислить cos125°cos5°+sin55°cos85°

Answer 1

cos125=cos(180-55)=-cos55

cos85=cos(90-5)=sin5

-cos55cos5+sin55sin5=-(cos55cos5-sin55sin5)=-(cos60)=-1/2