Question

在等差数列{an}中,a1=-5,a10=13,求s100.答

Answer 1

a1=-5,a10=13
a10=a1+9d=13
d=2,a1=-5
S100=-100*5-1/2*100*99*5=

Answer 2

根据性质，d=2
an=-5+2(n-1)
Sn=na1+n(n-1)/2 d
=-5n+n(n-1)=n^2-6n
S100=100^2-600=9400