Question

19．计算下面各题．

$\frac{1}{5}$+$\frac{1}{10}$=$\frac{3}{10}$$\frac{8}{15}$-$\frac{1}{5}$=$\frac{1}{3}$$\frac{2}{3}$+$\frac{4}{21}$=$\frac{6}{7}$$\frac{7}{9}$-$\frac{1}{4}$=$\frac{19}{36}$

$\frac{1}{2}$-$\frac{1}{7}$=$\frac{5}{14}$$\frac{5}{8}$-$\frac{1}{4}$=$\frac{3}{8}$$\frac{1}{2}$-$\frac{1}{3}$=$\frac{1}{6}$$\frac{1}{3}$+$\frac{1}{4}$=$\frac{7}{12}$．

Answer 1

分析 异分母分数相加（减），必须先通分，然后按照同分母分数相加（减）的法则进行运算，据此求出每个算式的值是多少即可．

解答 解：$\frac{1}{5}$+$\frac{1}{10}$
=$\frac{2}{10}+\frac{1}{10}$
=$\frac{3}{10}$

$\frac{8}{15}-\frac{1}{5}$
=$\frac{8}{15}-\frac{3}{15}$
=$\frac{1}{3}$

$\frac{2}{3}+\frac{4}{21}$
=$\frac{14}{21}+\frac{4}{21}$
=$\frac{6}{7}$

$\frac{7}{9}-\frac{1}{4}$
=$\frac{28}{36}-\frac{9}{36}$
=$\frac{19}{36}$

$\frac{1}{2}-\frac{1}{7}$
=$\frac{7}{14}-\frac{2}{14}$
=$\frac{5}{14}$

$\frac{5}{8}-\frac{1}{4}$
=$\frac{5}{8}-\frac{2}{8}$
=$\frac{3}{8}$

$\frac{1}{2}-\frac{1}{3}$
=$\frac{3}{6}-\frac{2}{6}$
=$\frac{1}{6}$

$\frac{1}{3}+\frac{1}{4}$
=$\frac{4}{12}+\frac{3}{12}$
=$\frac{7}{12}$
故答案为：$\frac{3}{10}$、$\frac{1}{3}$、$\frac{6}{7}$、$\frac{19}{36}$、$\frac{5}{14}$、$\frac{3}{8}$、$\frac{1}{6}$、$\frac{7}{12}$．

点评 此题主要考查了分数加法、分数减法的运算方法，要熟练掌握，解答此题的关键是要明确异分母分数加减法的运算法则．