1) sin(a+B)-cos(a-B) = sinacosB+ cosasinB - cosacosB - sinasinB =
(sinacosB- sinasinB )+ (cosasinB - cosacosB) = sina(cosB -sinB) +cosa(sinB - cosB) =
= sina(cosB -sinB) - cosa(cosB- sinB ) = (cosB -sinB) ( sina - cosa)
2) cos(a+B)-sin(a-B) = cosacosB - sinasinB- sinacosB+ cosasinB =
= (cosacosB + cosasinB) - (sinasinB + sinacosB) = cosa(cosB + sinB) - sina(sinB + cosB) =
= (cosB + sinB)( cosa -sina)
3) (cosB -sinB) ( sina - cosa)/ (cosB + sinB)( cosa -sina) = (sinB -cosB)/ ( sinB+ cosB )
Вот, вроде правильно
7,1 у- 1,2у+6=5у+10,5
5,9у+6=5у +10
5,9у-5у=10-6
0.9у=4
у=0.225