x2+y2-1小于0是xy+1-x-Y大于0的____条件?(充分非必要,必要非充分,充要,既不充分也不必要)

已知0小于等于x小于等于1 若x2+y2=3,xy=1,则x-y= 但为何不可以是1 因式分解x2+y2-xy-x-y+1 x2+y2-1小于0是xy+1-x-Y大于0的____条件?(充分非必要,必要非充分,充要,既不充分也不必要) 解方程组 ｛3x2-y2=8 {4x2+12xy+9y2=1 {x2+2xy+y2-4=0 x2+xy+y2=4 4x2-9y2=0 x2-2xy+y2-1=0还有{x2+y2=4xy-y2+4=0x2-----x的平方 y2------y的平方 还有{2x-3y=12x2-3xy+y2-4x+3y-3=0 设函数Z=f(x,y)=xy/x2+y2,则下列个结论中不正确的是（）A f(1,y/x)=xy/x2+y2 B f(1,x/y)=xy/x2+y2 C f(1/x,1/y)=xy/x2+y2 D f(x+y,x-y)=xy/x2+y2为什么选D,求详解 1*{x+y+xy=7{x2+y2+xy=132*{x2+y2=10{x+xy+y=7 已知x2+4y2+x2y2+1=6xy,则(x4-y4)/(2x2+xy-y2)*(2x-y)/(xy-y2)/(x2+y2/y)2= x2+y2 xy+x+y-1x y属于R 比较大小 x2+y2 xy+x+y-1 若实数x,y满足x2+y2+xy=1,则x+y的最大值是 若实数x,y满足x2+y2+xy=1,则x+y的最大值是 [2/xy/(1/x+1/y)2+(x2+y2)/(x2+2xy+y2)]*2x/(x-y) x2-2xy+y2-1/x-y-1 2x2+xy-3y2+x+4y-1因式分解 x2+y2+xy=1则x+y最大值 X2+Y2+XY=1,X+Y的最小值为 比较x2+y2与x+y+xy-1的大小? 已知x2+4y2+x2y2-6xy+1=0,求 x4-y4/2x-y 乘 2xy-y2/xy-y2 除以（x2+y2/xy）2乘1/x+y的值 (x-y)(x2+xy+y2)