an=a1+(n-1)d=5/6+(n-1)x(-1/6)=1-n/6sn=(a1+an)n/2=(5/6+1-n/6)n/2=(11/6-n/6)n/2=-5两边同时乘以12,得(11-n)n=-60n²-11n-60=0(n-15)(n+4)=0解得n=15 n=-4（舍去）an=1-n/6=1-15/6=-3/2