等差数列{an}中,a3+a10=5,求(1)S12.(2)a2+a11(3)a5+2a6+a6

问题描述:

等差数列{an}中,a3+a10=5,求(1)S12.(2)a2+a11(3)a5+2a6+a6

1)S12=6(a6+a7)=6(a3+a10)=30
2)a2+a11=a3+a10=5
3)写错了吧?a5+2a6+a93)a5+2a6+a9=2a6+2a7=2(a3+a10)=10