Question

等差数列{an}已知a3＋a8＝24,求S10

Answer 1

a3=a1+2d, a8=a1+7d,

so: a3+a8=2a1+9d=24

cause:Sn=a1*n+n(n-1)d/2

then: s10=a1*10+10(10-1)d/2

=10a1+45d=5(2a1+9d)=5*24=120

the answer is 120