等差数列an中,(1)若a4+a6+a15+a17=32,求S20,(2)若a7=15,求S13

问题描述:

等差数列an中,(1)若a4+a6+a15+a17=32,求S20,(2)若a7=15,求S13

等差数列an中,(1)若a4+a6+a15+a17=32,
a4+a6+a15+a17=32,
2(a1+a20)=32
a1+a20=16
S20=(a1+a20)×20/2=160,
(2)若a7=15,
a1+a13=30
S13=(a1+a13)×13/2=15×13=195