已知等差数列前三项为a,4,3a,前n项和为sn且sk=90,求a及k的值

问题描述:

已知等差数列前三项为a,4,3a,前n项和为sn且sk=90,求a及k的值

a,4,3a成等差数列
a+3a=8
a=2
d=4-a=2
2k+2k(k-1)/2=90
k^2+k-90=0
k=9或k=-10(舍)