由1) x=y+2+5 x=Y+7,
由2) 代入 x=Y+7 (y+7)y = (y+7-5)(y+2) y^2 +7y = (y+2)(y+2)
y^2+7y = y^2 +4y+4 y^2+7y-y^2-4y =4
3y=4 y=4/3 x=4/3+7=25/3^是什么意思y^2 =y*yy的2次方.写的话, 应该是y , 个小小的2字会写在y右上方.