四年级小数巧算
四年级小数巧算
掌握关键点:
一、加减法
凑整(尾巴凑整,加补凑整,基准数凑整)
二、乘法
1,找好朋友
2×5=10, 4×25=100, 8×125=1000
2,提取公因数
直接提 间接提 二次提
三、换元法
1,换短不换长,中间随便换
2,固定公式:有头无尾×无头有尾-有头有尾×无头无尾=头×尾
练习:
加补凑整
1.9+1.99+1.999+1.9999
=2-0.1+2-0.01+2-0.001+2-0.0001=...
尾巴凑整
36.14+36.86+35.45+35.55
=(36.14+36.86)+(35.45+35.55)=...
找好朋友
1.5×32×1.25×7
=1.5×4×8×1.25×7=(1.5×4)×(8×1.25)×7=...
25×3.6×2.4×1.25
=25×4×0.9×0.3×8×1.25=(25×4)×0.9×0.3×(8×1.25)=...
直接提取公因数
4.2×32-4.2×17+4.2×85
=4.2×(32-17+85)=4.2×100=420
间接提公因数(5.3、0.53、53最好提取中间数)
5.3×1.2+0.53×45+53×0.43
=5.3×1.2+5.3×4.5+5.3×4.3
=5.3×(1.2+4.5+4.3)=5.3×10=53
二次提取
0.47×1.2+0.86×1.2-1.33×0.2
=1.2×(0.47+0.86)-1.33×0.2
=1.2×1.33-1.33×0.2
=1.33×(1.2-0.2)=1.33×1=1.33
5.3+5.3×47+5.3×14+6.2×47
=5.3×(1+47+14)+6.2×47
=5.3×62+6.2×47
=5.3×62+62×4.7
=62×(5.3+4.7)=62×10=620
换元法
(0.1+2.8+3.6)×(2.8+3.6+4.7)-(2.8+3.6)×(0.1+2.8+3.6+4.7) 解:
方法1:换短不换长,中间随便换
短:2.8+3.6=x 中间:0.1+2.8+3.6=y
y×(x+4.7)-x×(y+4.7)
=y×x+y×4.7-y×x-x×4.7
=y×4.7-x×4.7
=4.7×(y-x )
=4.7×0.1=0.47
方法2:固定公式:有头无尾×无头有尾-有头有尾×无头无尾=头×尾 头:0.1
尾:4.7
有头无尾:0.1+2.8+3.6
无头有尾:2.8+3.6+4.7
有头有尾:0.1+2.8+3.6+4.7
无头无尾:2.8+3.6
头×尾:4.7×0.1
依据上述公式,原式=头×尾=4.7×0.1=0.47