9.8×4.2=（10-0.2）×4.2=10×4.2-0.2×4.2=42-0.84=41.16或9.8×4.2=（10-0.2）×（4+0.2）=10×4+10×0.2-0.2×4-0.2×0.2=40+2-0.8-0.04=42-0.8-0.04=41.2-0.04=41.16
