(1-1/18-1/19)*(1/17+1/18)+(1/18+1/19)*(1/17+1/18-1)
=[1-(1/18+1/19)]*(1/17+1/18)+(1/18+1/19)*(1/17+1/18-1)
=(1/17+1/18)- (1/18+1/19)*(1/17+1/18)+(1/18+1/19)*(1/17+1/18)-(1/18+1/19)
=(1/17+1/18)- (1/18+1/19)
=1/17+1/18- 1/18-1/19
=......
(1-1/18-1/19)*(1/17+1/18)+(1/18+1/19)*(1/17+1/18-1)
=[1-(1/18+1/19)]*(1/17+1/18)+(1/18+1/19)*(1/17+1/18-1)
=(1/17+1/18)- (1/18+1/19)*(1/17+1/18)+(1/18+1/19)*(1/17+1/18)-(1/18+1/19)
=(1/17+1/18)- (1/18+1/19)
=1/17+1/18- 1/18-1/19
=......