已知-1已知-1
已知-1换元.可设a=x+yb=x-y.易知,x=(a+b)/2.y=(a-b)/2且-1<a<12<b<3同时,2x+3y=(a+b)+[3(a-b)/2]=(5a-b)/2易知-5<5a<5-3<-b<-2∴-8<5a-b<3∴-4<(5a-b)/2<3/2即-4<2x+3y<3/2