a7=a1+6d=4；a19=a1+18d=2(a1+8d);a1=2d;8d=4;d=1/2;a1=1;∴an=a1+(n-1)d=1+(n-1)/2=(n+1)/2;(2)bn=1/n(n+1)/2=2/n(n+1)=2×[1/n-1/(n+1)];∴Sn=b1+b2+..+bn=2×[1-1/2+1/2-1/3+...+1/n-1/(n+1)]=2×n/(n+1)=2n/(n+1);
