an=a1+(n-1)da10=a1+9d a20=a1+19d a1=12 d=2an=12+2(n-1)=2n+10sn=(12+2n+10)*n/2=242 n=-22(舍) n=11
An=10+2n,当Sn=242，n=11
通项：an=2n+10;Sn=242,n=11.
