解:51×0.51━0.5
=51×(0.5+0.01)-0.5
=51×0.5+51×0.01-1×0.5
=(51-1)×0.5+0.51
=50×0.5+0.51
=25+0.51
=25.51
51×0.51━0.5
=51x0.5+0.51-0.5
=25.5+0.01
=25.51
51×0.51━0.5
=(50+1)×0.51━0.5
=50×0.51+1×0.51━0.5
=25.5+0.01
=25.51
51×0.51-0.51=(50+1)0.51-0.5=50×0.51+1×0.51-0.5=25.5+0.01=25.51