解:9.9x2.8+0.28=99x0.28+0.28=0.28x(99+1)=0.28x100=28
9.9*2.8+0.28=9.9*2.8+2.8*0.1=2.8*10=28
9.9X2.8+0.1X2.8=2.8X(9.9+0.1)=28
9.9*2.8+0.28*0.1=28