<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<script>
window.onload = function() {
var canvas = document.getElementById("canvas");
canvas.width = 800;
canvas.height = 800;
var context = canvas.getContext("2d");
context.arc(400,400,300,0.5*Math.PI,1.5*Math.PI,true);
context.moveTo(400,100);
context.arcTo(1200,400,400,700,(400-100)*dis(400,100,1200,400)/(1200-400));
context.stroke();
function dis(x1,y1,x2,y2){
return Math.sqrt((x1-x2)*(x1-x2),(y1-y2)*(y1-y2));
}
}
</script>
<body>
<canvas id="canvas" style="margin: 50px auto ;display: block; border:1px solid black;"></canvas>
</body>
</html>Math.sqrt()是返回数字的平方根，所以把22行的代码改成 return Math.sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));即可。建议你去查下两点间距离公式