2.5×3.2×1.25
=2.5×4×0.8×1.25
=（2.5×4）×（0.8×1.25）
=10×1
=10
1.98×9.9
=1.98×（10-0.1）
=1.98×10-1.98×0.1
=19.8-0.198
=19.602