3x+4y=16 5x-6y=33 这么做?

3x+4y=16 5x-6y=33 这么做? 已知-x+3y=5,求5(x-3y)²-8(x-3y)-5的值.请附理由,为什么这么做. {3(x+y)-4(x-y)=4 {x+y/2 + x-y/6=1 x+y/2+x-y/3=6,4(x+y)-3(x-y)=-20 {(x+y)/2+(x-y)/3=6 4(x+y)-3(x-y)=-20 4(x+y)-3(x-y)=-20,2/x+y+3/x-y=6 （5x+3y)(3y-5x)-(4x-y)(4y+x)= 6(x-y)-2(x+y)=14 3(x-y)+(x+y)=5 ｛4/(x+y)+6/(x-y)=3 ｛9/(x-y)-1/(x+y)=1 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 5(X-3Y)-6=2X+1,3(X+Y)=4(2X-Y)+16 3分之2(X-Y)-4分之X+Y=-1,6(X+Y)-4(2X-Y)=16