设数列(An)是等差数列,且a2=-6,a8=6,Sn是数列(An)的前n项和,求S4,S5,S6.
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![设数列(An)是等差数列,且a2=-6,a8=6,Sn是数列(An)的前n项和,求S4,S5,S6.](/uploads/image/z/1770586-34-6.jpg?t=%E8%AE%BE%E6%95%B0%E5%88%97%28An%29%E6%98%AF%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%2C%E4%B8%94a2%3D-6%2Ca8%3D6%2CSn%E6%98%AF%E6%95%B0%E5%88%97%28An%29%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%2C%E6%B1%82S4%2CS5%2CS6.)
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设数列(An)是等差数列,且a2=-6,a8=6,Sn是数列(An)的前n项和,求S4,S5,S6.
设数列(An)是等差数列,且a2=-6,a8=6,Sn是数列(An)的前n项和,求S4,S5,S6.
设数列(An)是等差数列,且a2=-6,a8=6,Sn是数列(An)的前n项和,求S4,S5,S6.
a8=a2+(8-2)d 所以:6=-6+6d 得:d=2 a1=a2-d=-6-2=-8 Sn=na1+[n(n-1)d/2]=-8n+n(n-1)=n^2-9n S4=16-36=-20 S5=25-45=-20 S6=36-54=-18