(1/8+1/24+1/48+1/80+1/120+1/168)*56=[1/(2*4)+1/(4*6)+1/(6*8)+1/(8*10)+1/(10*12)+1/(12*14)]*56=1/2*(1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12+1/12-1/14)*56=1/2*(1/2-1/14)*56=12