1.求函数图像的k值(y1-y2)/(x1-x2)
2.求与x轴平行线段的中点|x1-x2|/2
3.求与y轴平行线段的中点|y1-y2|/2
4.求任意线段的长√(x1-x2)^2+(y1-y2)^2 (注根号下(x1-x2)与(y1-y2)的平方和)
