原式=3x(1/4+1/8+1/16+1/32+1/64+1/128+1/256+1/512)
=3x[(1/2-1/4)+(1/4-1/8)+(1/8-1/16)+(1/16-1/32)+(1/32-1/64)+(1/64-1/128)+(1/128-1/256)+(1/256-1/512)]
=3x(1/2-1/512)
=3x255/512
=765/512
原式=3x(1/4+1/8+1/16+1/32+1/64+1/128+1/256+1/512)
=3x[(1/2-1/4)+(1/4-1/8)+(1/8-1/16)+(1/16-1/32)+(1/32-1/64)+(1/64-1/128)+(1/128-1/256)+(1/256-1/512)]
=3x(1/2-1/512)
=3x255/512
=765/512