解:原式=[1/(1x2)-1/(2x3)]/4+[1/(2x3)-1/(3x4)]/4+[1/(3x4)-1/(4x5)]/4+…+[1/(11x12)-1/(12x13)]/4=[1/(1x2)-1/(2x3)+1/(2x3)-1/(3x4)+1/(3x4)-1/(4x5)+…+1/(11x12)-1/(12x13)]/4=[1/(1x2)-1/(12x13)]/4=(77/156)/4=77/624
