section:nth-child(1) {
    margin: 0 120px 0 240px;
    border-left: 5px solid #ffda94;
}

section:nth-child(n+2):nth-child(-n+5) {
    margin: 0 120px 0 240px;
    border-left: 5px solid #ffda94;
}

section:nth-child(n+6):nth-child(-n+12) {
    margin: 0 100px 0 250px;
    border-right: 5px solid #ffda94;
}

section:nth-child(n+13):nth-child(-n+17) {
    margin: 0 120px 0 240px;
    border-left: 5px solid #ffda94;
}

section:nth-child(n+18):nth-child(-n+21) {
    margin: 0 100px 0 250px;
    border-right: 5px solid #ffda94;
}

/* MathML 样式 */
math{
    font-size: 25px;
    margin: 10px 0;
    text-align: center;
}

.math{
    text-align: center;
}