2分之1+4分之1+8分之1+16分之1+32分之1+64分之1+128分之1+256分之1
=(2/4+1/4)+1/8+1/16+1/32+1/64+1/128+1/256
=(6/8+1/8)+1/16+1/32+1/64+1/128+1/256
=(14/16+1/16)+1/32+1/64+1/128+1/256
=(30/32+1/32)+1/64+1/128+1/256
=(62/64+1/64)+1/128+1/256
=(126/128+1/128)+1/256
=254/256+1/256
=255/256