由杨辉三角形(m+n)^6=m^6+6m^5n+15m^4n^2+20m^3n^3+15m^2n^4+6mn^5+n^6
用二项式定理 系数是杨辉三角形中的第7行原式=m^6+6m^5n+15m^4n^2+20m^3n^3+15m^2n^4+6mn^5+n^6
