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一、比一比，看谁算得又快又准。
$5×2=$ $0.5×20=$ $0.05×200=$ $0.05×0.2=$
$125×8=$ $0.125×8=$ $1.25×0.8=$ $1.25×0.08=$
$5×2=$ $0.5×20=$ $0.05×200=$ $0.05×0.2=$
$125×8=$ $0.125×8=$ $1.25×0.8=$ $1.25×0.08=$
答案
$5×2=10$
$0.5×20=10$
$0.05×200=10$
$0.05×0.2=0.01$
$125×8=1000$
$0.125×8=1$
$1.25×0.8=1$
$1.25×0.08=0.1$
$0.5×20=10$
$0.05×200=10$
$0.05×0.2=0.01$
$125×8=1000$
$0.125×8=1$
$1.25×0.8=1$
$1.25×0.08=0.1$
二、在○内填“＞”“＜”或“=”。
3.4×0.8○3.4
3.4×1.8○3.4
7.1×1○7.1
93×0.1○93
24×0.99○24
0.8×1.25○1
348×0.8○348
4.3×1.1○4.3
1×0.83○1×1.2
3.4×0.8○3.4
3.4×1.8○3.4
7.1×1○7.1
93×0.1○93
24×0.99○24
0.8×1.25○1
348×0.8○348
4.3×1.1○4.3
1×0.83○1×1.2
答案
3.4×0.8 ＜ 3.4
3.4×1.8 ＞ 3.4
7.1×1 = 7.1
93×0.1 ＜ 93
24×0.99 ＜ 24
0.8×1.25 = 1
348×0.8 ＜ 348
4.3×1.1 ＞ 4.3
1×0.83 ＜ 1×1.2
3.4×1.8 ＞ 3.4
7.1×1 = 7.1
93×0.1 ＜ 93
24×0.99 ＜ 24
0.8×1.25 = 1
348×0.8 ＜ 348
4.3×1.1 ＞ 4.3
1×0.83 ＜ 1×1.2
三、数学门诊。（先判断对错，对的画“√”，错的画“×”，再将错误的改正）
答案
1. (×)
$\begin{array}{r} 542\\ ×\ \ 1.8\\ \hline 4336\\ 542\ \ \ \\ \hline 975.6\end{array}$
2. (×)
$\begin{array}{r} 0.46\\ ×\ 0.45\\ \hline 230\\ 184\ \ \ \\ \hline 0.2070\end{array}$
$\begin{array}{r} 542\\ ×\ \ 1.8\\ \hline 4336\\ 542\ \ \ \\ \hline 975.6\end{array}$
2. (×)
$\begin{array}{r} 0.46\\ ×\ 0.45\\ \hline 230\\ 184\ \ \ \\ \hline 0.2070\end{array}$
四、用简便方法计算下面各题。
0.5×0.47×0.8
4.2×2.6+1.3×1.5+0.13
0.5×0.47×0.8
4.2×2.6+1.3×1.5+0.13
答案
$0.5×0.47×0.8$
$=0.5×0.8×0.47$
$=0.4×0.47$
$=0.188$
$4.2×2.6+1.3×1.5+0.13$
$=4.2×2×1.3+1.3×1.5+1.3×0.1$
$=8.4×1.3+1.3×1.5+1.3×0.1$
$=1.3×(8.4+1.5+0.1)$
$=1.3×10$
$=13$
$=0.5×0.8×0.47$
$=0.4×0.47$
$=0.188$
$4.2×2.6+1.3×1.5+0.13$
$=4.2×2×1.3+1.3×1.5+1.3×0.1$
$=8.4×1.3+1.3×1.5+1.3×0.1$
$=1.3×(8.4+1.5+0.1)$
$=1.3×10$
$=13$
