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已知a1=5/6,d=-1/6,Sn=-5求n和an
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已知a1=5/6,d=-1/6,Sn=-5 求n和an
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答案和解析
an=a1+(n-1)*d;
sn=(a1+an)*n/2;
=(a1+a1+(n-1)d)*n/2;
=a1*n+(n-1)*n*d/2;
=5/6*n+(n-1)*n*(-1/6)/2=-5
5*2*n-(n-1)*n=-5*12
n^2-11n-60=0;
(n-15)(n+4)=0;
n=15,舍去（n=-4）n>0;
an=a1+14*d=5/6-14*(1/6)=-3/2
sn=(a1+an)*n/2;
=(a1+a1+(n-1)d)*n/2;
=a1*n+(n-1)*n*d/2;
=5/6*n+(n-1)*n*(-1/6)/2=-5
5*2*n-(n-1)*n=-5*12
n^2-11n-60=0;
(n-15)(n+4)=0;
n=15,舍去（n=-4）n>0;
an=a1+14*d=5/6-14*(1/6)=-3/2
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