设正项数列{an}的前n项和是sn,若{an}和{更号sn}都是等差数列,且公差相等,则a1+d=
问题描述：
设正项数列{an}的前n项和是sn,若{an}和{更号sn}都是等差数列,且公差相等,则a1+d=
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答
设公差为d,首项a1Sn=na1+n(n-1)d/2√Sn=√(S1)+(n-1)d=√(a1)+(n-1)d 平方Sn=a1+2√(a1)*(n-1)d+(n-1)^2d^2na1+n(n-1)d/2=a1+2√(a1)*(n-1)d+(n-1)^2d^2(n-1)a1+n(n-1)d/2=2√(a1)*(n-1)d+(n-1)^2d^2a1+nd/2=2√(a1)...