求等比数列-1,2,-4,8的第10项与前7项的和② 在等差数列an中 已知a1=-15 点(a3,a4)在直线x-y+2=0上,试求(1)通项an (2)若bn=-an求数列{bn}中所有正数项的和
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![求等比数列-1,2,-4,8的第10项与前7项的和② 在等差数列an中 已知a1=-15 点(a3,a4)在直线x-y+2=0上,试求(1)通项an (2)若bn=-an求数列{bn}中所有正数项的和](/uploads/image/z/5479129-1-9.jpg?t=%E6%B1%82%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97-1%2C2%2C-4%2C8%E7%9A%84%E7%AC%AC10%E9%A1%B9%E4%B8%8E%E5%89%8D7%E9%A1%B9%E7%9A%84%E5%92%8C%E2%91%A1+%E5%9C%A8%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%E4%B8%AD+%E5%B7%B2%E7%9F%A5a1%3D-15+%E7%82%B9%28a3%2Ca4%29%E5%9C%A8%E7%9B%B4%E7%BA%BFx-y%2B2%3D0%E4%B8%8A%EF%BC%8C%E8%AF%95%E6%B1%82%EF%BC%881%EF%BC%89%E9%80%9A%E9%A1%B9an+%EF%BC%882%EF%BC%89%E8%8B%A5bn%3D-an%E6%B1%82%E6%95%B0%E5%88%97%7Bbn%7D%E4%B8%AD%E6%89%80%E6%9C%89%E6%AD%A3%E6%95%B0%E9%A1%B9%E7%9A%84%E5%92%8C)
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