求数学达人,.1.等差数列{a n}的公差是正数,且a3a7=-12,a4+a6=-4,求他的前20项的和.2.已知数列{a n}为等差数列,前30项的和为50,前50项的和为30,求前80项的和.
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![求数学达人,.1.等差数列{a n}的公差是正数,且a3a7=-12,a4+a6=-4,求他的前20项的和.2.已知数列{a n}为等差数列,前30项的和为50,前50项的和为30,求前80项的和.](/uploads/image/z/693847-55-7.jpg?t=%E6%B1%82%E6%95%B0%E5%AD%A6%E8%BE%BE%E4%BA%BA%2C.1.%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%EF%BD%9Ba+n%EF%BD%9D%E7%9A%84%E5%85%AC%E5%B7%AE%E6%98%AF%E6%AD%A3%E6%95%B0%2C%E4%B8%94a3a7%3D-12%2Ca4%2Ba6%3D-4%2C%E6%B1%82%E4%BB%96%E7%9A%84%E5%89%8D20%E9%A1%B9%E7%9A%84%E5%92%8C.2.%E5%B7%B2%E7%9F%A5%E6%95%B0%E5%88%97%EF%BD%9Ba+n%EF%BD%9D%E4%B8%BA%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%2C%E5%89%8D30%E9%A1%B9%E7%9A%84%E5%92%8C%E4%B8%BA50%2C%E5%89%8D50%E9%A1%B9%E7%9A%84%E5%92%8C%E4%B8%BA30%2C%E6%B1%82%E5%89%8D80%E9%A1%B9%E7%9A%84%E5%92%8C.)
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