原式=(1/1001)*[(1/2)+(1/3)-(1/4)+(1/6)-(1/8)]
=(1/1001)*{[(1/2)+(1/3)+(1/6)]-[(1/4)+(1/8)]}
=(1/1001)*[1-(3/8)]
=(1/1001)*(5/8)
=5/8008