0.57×9.8+90.2×0.52
=（0.52+0.05）×9.8+90.2×0.52
=0.52×9.8+0.05×9.8+90.2×0.52
=（9.8+90.2）×0.52 +0.05×2×4.9
=100×0.52 + 0.1×4.9
=52+0.49
=52.49
.
3.99+（6-3/8×2/7）