1.设等差数列{an}的公差为-2,如果a1+a4+a7+……a97=50,则a3+a6+a9+……a99=?2.已知an为等差数列,S1=1,S4=5,则S6=?3.已知数列{an}为前n项和Sn=3n(41-n)/2,试求数列{|an|}前30项的和4.等差数列{an}中,若已知a2=4
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![1.设等差数列{an}的公差为-2,如果a1+a4+a7+……a97=50,则a3+a6+a9+……a99=?2.已知an为等差数列,S1=1,S4=5,则S6=?3.已知数列{an}为前n项和Sn=3n(41-n)/2,试求数列{|an|}前30项的和4.等差数列{an}中,若已知a2=4](/uploads/image/z/1759075-43-5.jpg?t=1.%E8%AE%BE%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E5%85%AC%E5%B7%AE%E4%B8%BA-2%2C%E5%A6%82%E6%9E%9Ca1%2Ba4%2Ba7%2B%E2%80%A6%E2%80%A6a97%3D50%2C%E5%88%99a3%2Ba6%2Ba9%2B%E2%80%A6%E2%80%A6a99%3D%3F2.%E5%B7%B2%E7%9F%A5an%E4%B8%BA%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%2CS1%3D1%2CS4%3D5%2C%E5%88%99S6%3D%3F3.%E5%B7%B2%E7%9F%A5%E6%95%B0%E5%88%97%EF%BD%9Ban%EF%BD%9D%E4%B8%BA%E5%89%8Dn%E9%A1%B9%E5%92%8CSn%3D3n%2841-n%29%2F2%2C%E8%AF%95%E6%B1%82%E6%95%B0%E5%88%97%7B%7Can%7C%7D%E5%89%8D30%E9%A1%B9%E7%9A%84%E5%92%8C4.%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%EF%BD%9Ban%EF%BD%9D%E4%B8%AD%2C%E8%8B%A5%E5%B7%B2%E7%9F%A5a2%3D4)
1.设等差数列{an}的公差为-2,如果a1+a4+a7+……a97=50,则a3+a6+a9+……a99=?2.已知an为等差数列,S1=1,S4=5,则S6=?3.已知数列{an}为前n项和Sn=3n(41-n)/2,试求数列{|an|}前30项的和4.等差数列{an}中,若已知a2=4
1.设等差数列{an}的公差为-2,如果a1+a4+a7+……a97=50,则a3+a6+a9+……a99=?
2.已知an为等差数列,S1=1,S4=5,则S6=?
3.已知数列{an}为前n项和Sn=3n(41-n)/2,试求数列{|an|}前30项的和
4.等差数列{an}中,若已知a2=4,a5=-1\2,求an=?
1.设等差数列{an}的公差为-2,如果a1+a4+a7+……a97=50,则a3+a6+a9+……a99=?2.已知an为等差数列,S1=1,S4=5,则S6=?3.已知数列{an}为前n项和Sn=3n(41-n)/2,试求数列{|an|}前30项的和4.等差数列{an}中,若已知a2=4
1)a1+a4+a7+……a97=50
a1+2d+a4+2d+a7+2d……a97+2d
=a1+a4+a7+……a97+2d*32
=50-128
=-78
2)S1=1,S4=5
a1=1
S4=a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4a1+6d=4+6d=5
d=1/6
S6=S4+a5+a6
=5+a1+4d+a1+5d
=7+9d=7+3/2=17/2
3)Sn=3n(41-n)/2 这是前n项和
所以前30项和s30=3*30(41-30)/2=495
4)a2=4,a5=-1\2
a5-a2=3d=-1/2-4=-9/2
d=-3/2
a1=a2-d=4+3/2=11/2
an=a1+(n-1)d=11/2-3/2n+3/2
=7-3n/2
1)a1+a4+a7+……a97=50
a1+2d+a4+2d+a7+2d……a97+2d
=a1+a4+a7+……a97+2d*32
=50-128
=-78
2)S1=1,S4=5
a1=1
S4=a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4a1+6d=4+6d=5
d=1/6
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1)a1+a4+a7+……a97=50
a1+2d+a4+2d+a7+2d……a97+2d
=a1+a4+a7+……a97+2d*32
=50-128
=-78
2)S1=1,S4=5
a1=1
S4=a1+a2+a3+a4
=a1+a1+d+a1+2d+a1+3d
=4a1+6d=4+6d=5
d=1/6
S6=S4+a5+a6
=5+a1+4d+a1+5d
=7+9d=7+3/2=17/2
3)Sn=3n(41-n)/2
所以前30项和s30=3*30(41-30)/2=495
4)a2=4,a5=-1\2
a5-a2=3d=-1/2-4=-9/2
d=-3/2
a1=a2-d=4+3/2=11/2
an=a1+(n-1)d=11/2-3/2n+3/2
=7-3n/2
注意运算
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