{an}为an=2n-30,sn是数列{|an|}的前n项和,s10=

题目

{an}为an=2n-30,sn是数列{|an|}的前n项和,s10=

答案

an=2n-30
a1=2*1-30=-28
a1<0
a10=2*10-30=-10
a10<0
s10=|a1|+|a2|+.+|a10|
s10=|(a1+a10)*10/2|
=|5(a1+a10)|
=|5*(-28-10)|
=|-190|
=190