(30-15)÷5
=15÷5
=3那它的通项公式是什么Sn=(a1+an)n/2=na1+n(n-1)d/2可得：10a1+10(10-1)x3/2=15+3010a1=-90a1=-9所以：an=a1+(n-1)d =-9+3n-3 =3n-12