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=-55*2*n-(n-1)*n=-5*12n^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;