等差数列中前三项分别为x,2x,5x-4得2x-x=5x-4-2x,x=2,前三项为2.4.6
前N项和为Sn=(2+2n)*n/2=n(n+1)
Sk=42得k=6
Tn=1/S1+/1/S2+1/S3+1/s4=1/2+1/6+1/12+1/20=1-1/5=4/5
Tk=1/S1+/1/S2+1/S3+...+1/sk=1-1/2+1/2-1/3+1/3-1/4+.+1/6-1/7=6/7
Tn=1/S1+/1/S2+1/S3+...+1/sn=1-1/2+1/2-1/3+1/3-1/4+.+1/n-1/(n+1)=1-1/(n+1)=n/(n+1)