证明S={y|y=2-x²,x∈R}无下界

证明S={y|y=2-x²,x∈R}无下界 若实数x,y满足2x²+3y²=1,S=3x²-2y²,则S的取值范围是多少 x²-4y² x+2y---------- ÷ --------分数线x²+2xy+y² x²+xyx²-4y² x+2y----------÷--------分数线x²+2xy+y² x²+xy(x+2y)(x-2y)x(x+y)=----------------(x+y)(x+y)(x+2y)(x-y)x=------(x+y)这样对了吗? 2x-y=2,求[(x²+y²)-(x-y)²+2y(x-y)]÷4y 若x²+xy-2y²=0,则x²+3xy+y²/x²+y² x²+xy-2y²=0则x²+3xy+y²/x²+y² x²-y²=1｛(x-y)²-2(x-y-2)=7 化简再求值[(x²+y²）²-4x²y²]÷(x²-y²）其实,x=2,y=2/3. 怎么算啊这个：X²+4X+Y²-2Y+5=0 则X²+Y²=?²²²²²²²²²²²²X²+4X+Y²-2Y+5X=0 则X²+Y²=? 4m²（2x-3y)²-m²（3x-2y)²=? 证明不定方程2x²-5y²=7无整数解 因式分解2x(x-y)四次方-x²(x-y)²=xy(y-x)² y=1/x²的奇偶性怎么证明? 已知x² -XY = 2,Y² -XY= 5,求x² - 4x Y+ Y² = 2x² + xY -3Y² = x² 已知x² -XY = 2,Y² -XY= 5,求x² - 4x Y+ Y² = 2x² + xY -3Y² = x² + xY- 2Y² =求x² - 4x Y+ Y² = 已知x²+4x+y²-2y+5=0,则x²+y²= x/y=2,求x²-3xy+y²/2x²-3xy+5y² 已知x/y=3,求x²+2xy-3y²/x²-2xy+y² x/y=2,求分式x²-xy+3y²/x²+xy+6y²