】已知等差数列{an}公差d=1/2,a2+a4……a100=80,S100=解析是a2+a4+……+a100=80a1+a3+……+a99=80-(50*1/2)=55【这是用的哪个公式啊?】S100=a1+a2+……+a99+a100=135或者S100=80*2-50*1/2=135
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![】已知等差数列{an}公差d=1/2,a2+a4……a100=80,S100=解析是a2+a4+……+a100=80a1+a3+……+a99=80-(50*1/2)=55【这是用的哪个公式啊?】S100=a1+a2+……+a99+a100=135或者S100=80*2-50*1/2=135](/uploads/image/z/399858-42-8.jpg?t=%E3%80%91%E5%B7%B2%E7%9F%A5%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E5%85%AC%E5%B7%AEd%3D1%2F2%2Ca2%2Ba4%E2%80%A6%E2%80%A6a100%3D80%2CS100%3D%E8%A7%A3%E6%9E%90%E6%98%AFa2%2Ba4%2B%E2%80%A6%E2%80%A6%2Ba100%3D80a1%2Ba3%2B%E2%80%A6%E2%80%A6%2Ba99%3D80-%2850%2A1%2F2%29%3D55%E3%80%90%E8%BF%99%E6%98%AF%E7%94%A8%E7%9A%84%E5%93%AA%E4%B8%AA%E5%85%AC%E5%BC%8F%E5%95%8A%3F%E3%80%91S100%3Da1%2Ba2%2B%E2%80%A6%E2%80%A6%2Ba99%2Ba100%3D135%E6%88%96%E8%80%85S100%3D80%2A2-50%2A1%2F2%3D135)
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