Awesome! It works)
Thank you very much!
Try
t={1,3,5,12}; n=4; Plot[1/x*Sum[1/(1+x*t[[i]]),{i,1,n}]-n*t[[n]]/((1+x*t[[n]])*Log[1+x*t[[n]]]),{x,-1,1}] FindRoot[1/x*Sum[1/(1+x*t[[i]]),{i,1,n}]==n*t[[n]]/((1+x*t[[n]])*Log[1+x*t[[n]]]),{x,1/7}]
