/ 等腰三角形（箭头朝上）； /
div1{
width: 0;
height: 0;
border-bottom: 100px solid cyan;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
}
/ 等腰三角形（箭头朝下）； /
div2{
width: 0;
height: 0;
border-top: 100px solid cyan;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
}