8.4×2.5+7.5×1.2的简便和3.7×15+21×405的简便计算？
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题一：
8.4×2.5+7.5×1.2
=8.4×2.5+2.5×3×1.2
=2.5×8.4+2.5×3.6
=2.5×(8.4+3.6)
=2.5×12
=2.5×4×3
=10×3
=30
题二：
3.7×15+21×405
=3.7×(10+5)+(20+1)×(400+5)
=37+18.5+20×400+20×5+400+5
=55.5+8000+100+400+5
=55.5+5+8500
=60.5+8500
=8560.5
8.4×2.5+7.5×1.2
=8.4×2.5+2.5×3×1.2
=2.5×8.4+2.5×3.6
=2.5×(8.4+3.6)
=2.5×12
=2.5×4×3
=10×3
=30
题二：
3.7×15+21×405
=3.7×(10+5)+(20+1)×(400+5)
=37+18.5+20×400+20×5+400+5
=55.5+8000+100+400+5
=55.5+5+8500
=60.5+8500
=8560.5
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8.4×2.5+7.5×1.2
=8.4*2.5+(2.5*3*1.2)
=8.4*2.5+2.5*3.6
=2.5*(8.4+3.6)
=2.5*12
=2.5*(10+2)
=2.5*10+2.5*2
=25+5
=30
=8.4*2.5+(2.5*3*1.2)
=8.4*2.5+2.5*3.6
=2.5*(8.4+3.6)
=2.5*12
=2.5*(10+2)
=2.5*10+2.5*2
=25+5
=30
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8.4×2.5+7.5×1.2
=2.1×（4×2.5）+7.5×2×0.6
=2.1×10+15×0.6
=21+9
=30
=2.1×（4×2.5）+7.5×2×0.6
=2.1×10+15×0.6
=21+9
=30
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