3.6x2.5
=0.9×（4×2.5）
=0.9×10
=9
(12.4+4)x2.5
=12.4×2.5＋4×2.5
=31＋10
=41
1.02x2.5
=（1＋0.02）×2.5
=2.5×1＋2.5×0.02
=2.5＋0.05
=2.55
12.4x4x2.5
=12.4×（4×2.5）
=12.4×10
=124
4.38x8.02+8.02x4.62+8.02
=8.02×（4.38＋4.62＋1）
=8.02×10
=80.2
2.9x2.9+2.9x7.1
=2.9×（2.9＋7.1）
=2.9×10
=29
199.4x19.93-199.3x19.92…………变一变,就简便
=199.4×19.93－19.93×199.2
=19.93×（199.4－199.2）
=19.93×0.2
=3.986
5.4x3.8-6.5x4.4+2.7x5.4
=5.4×3.8＋2.7×5.4－6.5×4.4
=5.4×（3.8＋2.7）－6.5×4.4
=5.4×6.5－6.5×4.4
=6.5×（5.4－4.4）
=6.5×1
=6.5