a1=5/6.d=-1/6.Sn=-5,求n和An

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;