x+y=80 5x+2.5y≤290 1.5x+3.5y≤212

x+y=80 5x+2.5y≤290 1.5x+3.5y≤212 {6(x-y)-7(x+y)=21 {2(x-y)-5(x+y)=-1 (x+y)/6+(x-y)/10=1,(x+y)/6-(x-y)/10=5 (x-y)/(x+y)=3求( 3x-2y-1)/(x+y-5) z=0.015x*x+y*x*0.05+(1-x-y)*2.5*(x+y) x>0.35.xy>0.70x+y {(x+y)/2+(x-y)/3=1,(x+y)-5(x-y)=2.求x=?,y=? x=3,y=-x+5(x＞1) y=-x+5(x≤1),y=? 若x,y满足条件 x+y≤3,y≤x+1,x-3y≤3,则z=3x+5y的最大值是, 化简[(3x+4y)^2-(2x+y)(2x-y)+(-x+y)(5x-y)]除以-2y,其中x=-1,y=1 （x+y-5)*（x+y-5)+（x-y+3)=0 x*x-y*y 已知:3x-5y=0,求x/y,x-y/y,x-y/x 分式的加减:[(x/y-y/x)除以（x+y）+x（1/y-1/x)]除以1+x/y,其中x=5,y=2 1/x-1/y=x,求5y+xy-5y/y-xy-x=? 数学不等式：变量x,y满足约束条件「x-y≥0,x+y≤0,x+2y≥1」,求z=5x+y的最大值变量x,y满足约束条件「x-y≥0,x+y≤0,x+2y≥1」,求z=5x+y的最大值,有图最好!变量x,y满足约束条件「x-y≥0,x+y≤1,x+2y≥1」， 5(x-y)³-3（x-y）²+7(x-y)-5(x-y)³+（x-y）²-5（x-y）,其中,x-y=1/3.5(x-y)³-3（x-y）²+7(x-y)-5(x-y)³+（x-y）²-5（x-y）,其中,x-y=1/3先化简,再求值； y'/y=1/x 先化简,再求值[(2x+y)^2-(2x+y)(2x-y)-y(5x+y)]÷(1/2y),其中x-y=2 x+y=5; x^y+y^x=17