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.
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![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.](/uploads/image/z/3960481-49-1.jpg?t=%E5%87%A0%E9%81%93%E6%95%B0%E5%AD%A6%E9%A2%98%28%E6%9C%89%E5%85%B3%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%291.%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%2CS20%3D170%2C%E5%88%99a6%2Ba9%2Ba11%2Ba16%3D2.%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%2CS9%3D18%2CSn%3D240%2C%E4%B8%94a%28n-4%29%3D30%2C%E6%B1%82n%28n%E2%89%A55%293.%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%2Cd%3D1%2CS99%3D99%2C%E6%B1%82a3%2Ba6%2Ba9%2B%E2%80%A6%E2%80%A6%2Ba99%E7%9A%84%E5%80%BC.)
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.
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.
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.
S20=10(a1+a20)=10(a10+a11)=170
a1+a20=a10+a11=17
a6+a9+a11+a16= 2(a1+a20)=34
S9=(a1+a9)+(a2+a8)+…+(a4+a6)+a5
=9a5
a5=2
a1+an=a5+a(n-4)=32
Sn=(a1+an)*d/2
d=2Sn/(a1+an)=15
1.a6+a9+a11+a16=2(a10+a11)=2(a1+a20)=34
2. a1+a9=2a5=4
a1+an=480/n
a5+a(n-4)=480/n
a5=2 a(n-4)=30 带入得n=15
3.s99=99 => a1+a99=2 =>a50=1 =>2a51=4
a3+a6+a9+...+a99=15*(2*a56)=60