Question

3．計(jì)算下面各題，能簡(jiǎn)算的要簡(jiǎn)算．
$\frac{5}{6}$×$\frac{12}{21}$×$\frac{7}{15}$；       $\frac{5}{6}$×$\frac{8}{15}$+$\frac{1}{6}$×$\frac{8}{15}$；         1+$\frac{2}{9}$+$\frac{7}{9}$；        $\frac{2}{5}$-$\frac{1}{6}$+$\frac{1}{2}$．

Answer 1

分析 ①$\frac{5}{6}$×$\frac{12}{21}$×$\frac{7}{15}$，先約分，再按照分?jǐn)?shù)乘法的計(jì)算法則計(jì)算； 
 ②$\frac{5}{6}$×$\frac{8}{15}$+$\frac{1}{6}$×$\frac{8}{15}$，運(yùn)用乘法分配律簡(jiǎn)算；  
③1+$\frac{2}{9}$+$\frac{7}{9}$，運(yùn)用加法結(jié)合律簡(jiǎn)算； 
④$\frac{2}{5}$-$\frac{1}{6}$+$\frac{1}{2}$，先通分，再按照同分母分?jǐn)?shù)加、減法的計(jì)算法則計(jì)算．

解答 解：①$\frac{5}{6}$×$\frac{12}{21}$×$\frac{7}{15}$
=$\frac{10}{21}×\frac{7}{15}$
=$\frac{2}{9}$；
 
 ②$\frac{5}{6}$×$\frac{8}{15}$+$\frac{1}{6}$×$\frac{8}{15}$
=$（\frac{5}{6}+\frac{1}{6}）×\frac{8}{15}$
=$1×\frac{8}{15}$
=$\frac{8}{15}$； 
 
③1+$\frac{2}{9}$+$\frac{7}{9}$
=1+（$\frac{2}{9}+\frac{7}{9}$）
=1+1
=2； 

④$\frac{2}{5}$-$\frac{1}{6}$+$\frac{1}{2}$
=$\frac{12}{30}-\frac{5}{30}+\frac{15}{30}$
=$\frac{7}{30}+\frac{15}{30}$
=$\frac{22}{30}$
=$\frac{11}{15}$．

點(diǎn)評(píng) 此題考查的目的是理解掌握分?jǐn)?shù)四則混合運(yùn)算的順序以及它們的計(jì)算法則，并且能夠靈活選擇簡(jiǎn)便方法進(jìn)行計(jì)算．