样例：http://www.zhaojz.com.cn/demo/draw9.html

复制代码 代码如下:

//画切线

//point 圆外的一点

//dot 圆心

//r 半径

function drawCircleTangent(point, dot, r){

//画辅助线-start

var color = 'DarkRed'; //切线的颜色

var color2 = "#ccc"; //其它辅助线的颜色

drawLine(dot, [dot[0]+9*r,dot[1]], {color: color2}); // 延长圆心所在的水平线

drawLine(dot, [dot[0],dot[1]-4*r], {color: color2}); // 画出圆心所在的垂直线

drawPoint({

pw:2,ph:2,color:'DarkRed',point: [dot[0]+9*r,dot[1],'x']

});

drawPoint({

pw:2,ph:2,color:'DarkRed',point: [dot[0],dot[1]-4*r,'y']

});

drawLine(point, [point[0],dot[1]], {color: color2}); // 画point到x轴的垂直线

drawLine(point, dot, {color: color2}); // 连接point与dot

drawLine([point[0]-2*r, point[1]], [point[0]+2*r, point[1]], {color: color2}); //画point所在的水平线

//画辅助线-end

//point.push('point');

drawPoint({

pw:2,ph:2,color:'DarkRed',point: point

});

//dot.push('centre');

var r_square = Math.pow(r,2); // r的平方

var point_v = point[1]-dot[1]; //point到x轴的距离的平方

var point_h = point[0]-dot[0]; //point到y轴的距离的平方

var c_square = Math.pow(point_v,2) + Math.pow(point_h,2); //point到圆心的距离的平方

var c = Math.sqrt(c_square); //point到圆心的距离

var sinA = Math.abs(point_v)/c; //sinA

var cosA = Math.abs(point_h)/c; //cosA

var b_square = c_square-r_square; //point到切点的距离的平方

var b = Math.sqrt(b_square); //point到切点的距离

var sinB = b/c; //sinB

var cosB = r/c; //cosB

//以圆心为坐标圆点，确定point所在的象限

var quadrant = 1; //默认值

var pm_h = point_h == 0? point_h: point_h/Math.abs(point_h); //水平方向

var pm_v = point_v == 0? point_v: point_v/Math.abs(point_v); //垂直方向

var hv = pm_h*pm_v; //相乘，-1时point在一三象限，+1时point在二四象限，0时point在轴上

switch(hv){

case 1:

if((pm_h+pm_v)==-2){

quadrant = 2; //第二象限

}else{

quadrant = 4; //第四象限

}

break;

case -1:

if((pm_h-pm_v)==-2){

quadrant = 3; //第三象限

}

break;

case 0:

if((pm_h+pm_v)==-1){ //point在x轴的负半轴或者y轴的正半轴时，断定point在第二象限

quadrant = 2;

}

if((pm_h+pm_v)==1){ //point在x轴的正半轴或者y轴的负半轴时，断定point在第四象限

quadrant = 4;

}

break;

default:

}

var sinC = 0;

var conC = 0;

var sinD = 0;

var conD = 0;

switch(quadrant){

case 1:

sinC = cosB*cosA+sinB*sinA; //sinC = sin(90+(B-A)) = cos(B-A) = cosB*cosA+sinB*sinA

conC = -(sinB*cosA-cosB*sinA); //cosC = cos(90+(B-A)) = -sin(B-A) = -(sinB*cosA-cosB*sinA)

sinD = -(cosA*cosB-sinA*sinB); //sinD = sin(270-(A+B))

conD = -(sinA*cosB+cosA*sinB); //conD = cos(270-(A+B))

break;

case 2:

sinC = -(cosB*cosA-sinB*sinA); //sinC = sin(-90+(A+B))

conC = sinA*cosB+cosA*sinB; //conC = cos(-90+(A+B))

sinD = cosA*cosB+sinA*sinB; //sinD = sin(90+(A-B))

conD = -(sinA*cosB-cosA*sinB); //conD = cos(90+(A-B))

break;

case 3:

sinC = -(cosA*cosB+sinA*sinB); //sinC = sin(-90+(A-B))

conC = -(sinA*cosB-cosA*sinB); //conC = cos(-90+(A-B))

sinD = (cosA*cosB-sinA*sinB);

conD = sinA*cosB+cosA*sinB;

break;

case 4:

sinC = cosA*cosB-sinA*sinB;

conC = -(sinA*cosB+cosA*sinB)

sinD = -(cosA*cosB+sinA*sinB); //sinD = sin(270+(A-B))

conD = (sinA*cosB-cosA*sinB); //conD = cos(270+(A-B))

break;

default:

}

var tangentPointA = [point[0]+b*conC, point[1]+b*sinC]; //切点A位置

drawLine(point, tangentPointA, {color: color}); //画出切线

drawLine(dot, tangentPointA, {color: color2}); //连接圆点与切点

//drawArc(point, 17, (quadrant ==1 ||quadrant==4?180:0)-(quadrant ==2 ||quadrant==3?(-1):1)*Math.asin(sinC)*180/Math.PI, 0);

var tangentPointB = [point[0]+b*conD, point[1]+b*sinD]; //切点B位置

drawLine(point, tangentPointB, {color: color}); //画出切线

drawLine(dot, tangentPointB, {color: color2}); //连接圆点与切点

//drawArc(point, 27, (quadrant ==1 ||quadrant==4?180:0)-(quadrant ==2 ||quadrant==3?(-1):1)*Math.asin(sinD)*180/Math.PI, 0);

drawPoint({ //描切点

pw:3,ph:3,color:'DarkRed',point: tangentPointA

});

drawPoint({ //描切点

pw:3,ph:3,color:'DarkRed',point: tangentPointB

});

//画辅助弧

//(quadrant ==1 ||quadrant==4?360:0)

drawArc(point, b, 60, (quadrant ==1 ||quadrant==4?180:0)-(quadrant ==2 ||quadrant==3?(-1):1)*Math.asin(sinC)*180/Math.PI-5);

}