原式=1/32(1+1/3+1/6+1/10+1/15)
=2/32(1/2+1/6+1/12+1/20+1/30)
=1/16(1-1/2+1/2-1/3+1/3-1/4+……-1/6)
=1/16(1-1/6)
=5/96