等差数列|an|(n是下标),a1（1是下标）=0,a5+a9(5,9是下标)=98,Sn(n是下标)为其前n项和,则S9(9为下标)等于

Question

等差数列|an|(n是下标),a1（1是下标）=0,a5+a9(5,9是下标)=98,Sn(n是下标)为其前n项和,则S9(9为下标)等于

Answer 1

设等差为b
an(n为下标)=a1(1为下标)+(n-1)b
a5(5为下标)+a9(9为下标)=a1(1为下标)+4b+a1(1为下标)+8b
=2*a1(1为下标)+12b
=98
12b=98
b=49/6(这是个分数,我不会打分数的数)
s9(9为下标)=a1(1为下标)+a2(2为下标)+.+a9(9为下标)
=a1(1为下标)+a1(1为下标)+1b+...+a1(1为下标)+8b
=9*a1(1为下标)+36b
=36b(a1(1为下标)=0)
=36*(49/6)
=6*49
=294
分给我吧!看在我是第一个回答的