y=-x^2+3x+5=-(x^2-3x+9/4）+29/429=-（x-3/2)^2+29/4 这两步是怎么得来的,

已知x-y/x+y=3,求代数式5(x-y)/x+y-x+y/2(x-y) 3^x+2x=y,x 若2x-3y+4=0则x（x*x-1)+x(5-x*x)-6y+7 先化简,再求值 [2/3x - 2/x+y（x+y/3x -x-y）]*x/x-y,其中5x+3y=0 x+y/2+x-y/6=5 2(x+y)-3(x-y)=-2 先化简,再求值:[3x-2/x+y(x+y/3x-x-y)]/(x-y/x),其中5x+3y=0 3(x+y)+2(x-y)=36 4(x+y)-5(x-y)=2 (x-y)/(x+y)=3求( 3x-2y-1)/(x+y-5) 4(x-y)-3(x-y)=9 5(x-y)+2(x-y)=11 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 6(x-y)-2(x+y)=14 3(x-y)+(x+y)=5 用递归法求y=x-(x*x*x/3!)+(x*x*x*x*x/5!)-(x*x*x*x*x*x*x/7!)+. 函数,y=3x/(x^2+x+1) ,x 函数y=3x/(x^2+x+1) （x 先化简再求值(x-y)(x-2y)+(x-2y)(x-3y)-2(x-3y)(x-4y) x=4 y=5 2x+3x-5y= (x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)= (x+y)(x-y)+(x-y)平方-(x-y)(3x-y) 其中x=-2 y=3分之一