设等差数列的第一项为a1,公差为d,则Sn=na1﹢n(n-1)d/2
S5=5a1﹢5(5-1)d/2=5a1﹢10d=10
S10=10a1﹢10(10-1)d/2=10a1﹢45d=-5
d=-1,a1=4
Sn=4n+n(n-1)x(-1)/2
=4n-(n²-n）/2
=(9n-n²）/2