1 、3x-2x²-2=-2(x²-3/2x+1)
=-2[x²-3/2x+(3/4)²-(3/4)²+1]
=-2(x-3/4)²-7/8
不论x取何值,(x-3/4)²均大于或等于0,所以
当(x-3/4)²＝0时代数式的值最大
即x=3/4
2、A=2x²-4x-1＝2（x²-2x+1)-3
＝2(x-1)²-3
B=x²-2x-4=(x²-2x+1)-5
=(x-1)²-5
因为(x-1)²大于或等于0,(x-1)²大于或等于0
所以A大于B
3、应是x²-7xy+12y²＝0
分解因式得（x-3y)(x-4y)=0
x-3y=0或x-4y=0
x=3y或x=4y
当x=3y时,x-y/x+2y=3y-y/3y+2y=2/5
当x=4y时,x-y/x+2y=4y-y/4y+2y=1/2