等差数列—3,1,5的前几项和是150

设等差数列为{an}
1-(-3)=5-1=4,数列是以-3为首项,4为公差的等差数列
an=-3+4(n-1)=4n-7
Sn=(a1+an)n/2
=(-3+4n-7)n/2
=2n²-5n
令2n²-5n=150
2n²-5n-150=0
(2n+15)(n-10)=0
n=-15/2(n为正整数,舍去)或n=10
数列前10项的和是150.