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设等差数列的首项a1,公差d前2n+1项时S奇=a1+a3+a5+.+a(2n+1) =a1*(n+1)+(n+1)(n+1-1)*2d/2 =a1*(n+1)+n(n+1)dS偶=a2+a4+a6+...+a(2n) =a2*n+n*(n-1)*2d/2 =(a1+d)*n+n(n-1)d =a1*n+n*(n-1+1)...
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