几道数学题(有关等差数列)
1.等差数列{an},S20=170,则a6+a9+a11+a16=
2.等差数列{an},S9=18,Sn=240,且a(n-4)=30,求n(n≥5)
3.等差数列{an},d=1,S99=99,求a3+a6+a9+……+a99的值.