Question

等差数列{an}的前n项和为Sn,S15=225,S9-S5=54,求通项

Answer 1

S15=15a1+1/2*15*14*d=225

即a1+7d=15.(1)

S9-S5=(9a1+36d)-(5a1+10d)=4a1+26d=54

即2a1+13d=27.(2)

(1)(2)得a1=-6,d=3.

故an=a1+(n-1)d=-6+3(n-1)=3n-9