圆与x轴相切,半径就等于圆心纵坐标,如果设圆心(a,b),那么半径就是|b|,标准方程当然就是(x-a)²+(y-b)²=b²
(本来是(x-a)²+(y-b)²=r²,但r=|b|)
x²+y²+Dx+Ey+F=0
(x+D/2)²+(y+E/2)²=E²/4+(D²-4F)/4
同样的道理半径的平方E²/4+(D²-4F)/4等于E²/4,所以D²-4F=0
![](https://iknow-pic.cdn.bcebos.com/9922720e0cf3d7ca56931e05f21fbe096b63a95f?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)