Question

a1=5/6,d=-1/6,Sn=-5,求n及an

Answer 1

an=a1+(n-1)*d;

sn=(a1+an)*n/2;

=(a1+a1+(n-1)d)*n/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;