lǎo / shī / yòng / yáng / huī / sān / jiǎo / jiǎng / jiě / èr / xiàng / shì / xì / shù / de / guī / lǜ

老师用杨辉三角讲解二项式系数的规律。

The teacher used Pascal's triangle to explain the pattern of binomial coefficients.