4×6分之2+6×8分之2+8×10分之2+10×12分之2+12×14分之2
=(1/4-1/6)+(1/6-1/8)+(1/8-1/10)+(1/10-1/12)+(1/12-1/14)
=1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12+1/12-1/14
=1/4-1/14
=14/56-4/56
=5/28