Question

15．脫式計算．
2÷（$\frac{2}{5}$×$\frac{1}{4}$）
$\frac{3}{8}$÷$\frac{1}{10}$×$\frac{5}{7}$
$\frac{1}{5}$+$\frac{2}{9}$÷$\frac{5}{9}$
$\frac{3}{5}$×$\frac{6}{7}$×$\frac{5}{21}$．

Answer 1

分析 ①先算小括號里面的乘法，最后算除法．
②先算除法再算乘法．
③先算除法再算加法．
④乘法交換律和乘法結(jié)合律計算．

解答 解：①2÷（$\frac{2}{5}$×$\frac{1}{4}$）
=2÷$\frac{1}{10}$
=20

②$\frac{3}{8}$÷$\frac{1}{10}$×$\frac{5}{7}$
=$\frac{3}{8}$×10×$\frac{5}{7}$
=$\frac{75}{28}$

③$\frac{1}{5}$+$\frac{2}{9}$÷$\frac{5}{9}$
=$\frac{1}{5}$+$\frac{2}{9}$×$\frac{9}{5}$
=$\frac{1}{5}+\frac{2}{5}$
=$\frac{3}{5}$

④$\frac{3}{5}$×$\frac{6}{7}$×$\frac{5}{21}$
=$\frac{3}{5}×\frac{5}{21}×\frac{6}{7}$
=$\frac{1}{7}×\frac{6}{7}$
=$\frac{6}{49}$

點評 本題主要考查分數(shù)四則混合運算的計算順序．要靈活運用簡便方法計算．