原式=(1-1/20)*(200-1)*(1-1/20)
=(1-1/20)^2*(200-1)
=(1+1/400-1/10)*(200-1)
=200+1/2-20-1-1/400+1/10
=180-1/2+1/10-1/400
=180-0.5+0.1-0.0025
=179.5975