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hello，大家好，最近在看前端的八股，里面有这样一道题，如何用CSS画出三角形？我想以这个题为例，仔细讲一下这个题的技巧，以及对这道题拓展一下，即如何画出圆形和椭圆形？
首先，如何用CSS画一个三角形？
步骤：
1. 设置宽度、高度为0的一个div盒子；
2. 为了方便理解，将盒子的4个边框分别设置一样的宽度border，不同的颜色；
width: 0px; height: 0px; border-top: 100px solid red; border-right: 100px
solid green; border-bottom: 100px solid yellow; border-left: 100px solid blue;
3. 上图是四个三角形，如果我们只需要其中一个三角形，要做的就是用transparent将其他三个边框隐藏掉，就能看到效果了；
border-top: 100px solid red; border-right: 100px solid transparent;
border-bottom: 100px solid transparent; border-left: 100px solid transparent;
ps：如果想要右边绿色的三角形（方向朝左的三角形），只需将border-right的颜色transparent改为green；
border-top: 100px solid transparent; border-right: 100px solid green;
border-bottom: 100px solid transparent; border-left: 100px solid transparent;
如果想要下边黄色的三角形（方向朝上的三角形），只需将border-bottom的颜色transparent改为yellow；
border-top: 100px solid transparent; border-right: 100px solid transparent;
border-bottom: 100px solid yellow; border-left: 100px solid transparent;
如果想要左边蓝色的三角形（方向朝右的三角形），只需将border-left的颜色transparent改为blue；
border-top: 100px solid transparent; border-right: 100px solid transparent;
border-bottom: 100px solid transparent; border-left: 100px solid blue;
大家可能会有疑问，为什么div的宽度和高度要设置为0px呢？
ok，如果div的宽度和高度不为0，画出来将是下图这样的：
如果对三角形的样式有特殊的要求，可以通过动画效果来实现，即用transform：rotate 旋转来实现
拓展
如何用CSS画出圆、椭圆以及圆角的长方形？
提示：通过改变border-radius的比例，可以显示不同的图形。
