Все на картинке
S_(n)=a1+a2+...+a_(n)= =(1/(4*7))+(1/(7*11))+ ... +(1/((3n+1)(3n+4)))= =(1/3)*((1/4)-(1/7))+(1/3)*((1/7)-(1/11))+... +(1/3)*((1/(3n+1))-(1/(3n+4)))= =(1/3)*(1/4 - (1/(3n+4)))= S=lim_(n →∞)S_(n)=1/12