等差数列{an}的首项为a1>0 ,前n项和为Sn,当l≠m时,Sm=Sl,问n为何值时,Sn最大∵Sm=SL,∴m/2[2a1+(m-1)d]=L/2[2a1+(L-1)d],∴d=(-2a1)/(L+m-1)……………………………………这两步怎么化的,
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![等差数列{an}的首项为a1>0 ,前n项和为Sn,当l≠m时,Sm=Sl,问n为何值时,Sn最大∵Sm=SL,∴m/2[2a1+(m-1)d]=L/2[2a1+(L-1)d],∴d=(-2a1)/(L+m-1)……………………………………这两步怎么化的,](/uploads/image/z/4562662-22-2.jpg?t=%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E9%A6%96%E9%A1%B9%E4%B8%BAa1%3E0+%2C%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E5%BD%93l%E2%89%A0m%E6%97%B6%2CSm%3DSl%2C%E9%97%AEn%E4%B8%BA%E4%BD%95%E5%80%BC%E6%97%B6%2CSn%E6%9C%80%E5%A4%A7%E2%88%B5Sm%3DSL%2C%E2%88%B4m%2F2%5B2a1%2B%EF%BC%88m-1%EF%BC%89d%5D%3DL%2F2%5B2a1%2B%28L-1%29d%5D%2C%E2%88%B4d%3D%EF%BC%88-2a1%EF%BC%89%2F%EF%BC%88L%2Bm-1%EF%BC%89%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E2%80%A6%E8%BF%99%E4%B8%A4%E6%AD%A5%E6%80%8E%E4%B9%88%E5%8C%96%E7%9A%84%2C)
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