题目内容

14.怎样计算简便就怎样算
$\frac{1}{5}$×16÷$\frac{8}{15}$            
 $\frac{2}{21}$÷$\frac{5}{4}$+$\frac{2}{21}$×$\frac{1}{5}$           
$\frac{4}{9}$÷($\frac{5}{9}$+$\frac{25}{21}$×$\frac{7}{15}$)
1.2×$\frac{2}{21}$+2.3×$\frac{2}{21}$-2.5×$\frac{2}{21}$          
(1÷2)÷(2÷3)÷(3÷4)÷(5÷6)

分析 (1)先算乘法,再算除法;
(2)、(4)利用乘法律进行计算;
(3)先算小括号内的乘法,再算小括号内的加法,最后算括号外的除法;
(5)先算各个小括号内的除法,再利用除法性质进行计算.

解答 解:(1)$\frac{1}{5}$×16÷$\frac{8}{15}$
=$\frac{16}{5}$×$\frac{15}{8}$
=6

(2)$\frac{2}{21}$÷$\frac{5}{4}$+$\frac{2}{21}$×$\frac{1}{5}$
=$\frac{2}{21}$×($\frac{4}{5}$+$\frac{1}{5}$)
=$\frac{2}{21}$×1
=$\frac{2}{21}$

(3)$\frac{4}{9}$÷($\frac{5}{9}$+$\frac{25}{21}$×$\frac{7}{15}$)
=$\frac{4}{9}$÷($\frac{5}{9}$+$\frac{5}{9}$)
=$\frac{4}{9}$×$\frac{9}{10}$
=$\frac{2}{5}$

(4)1.2×$\frac{2}{21}$+2.3×$\frac{2}{21}$-2.5×$\frac{2}{21}$
=$\frac{2}{21}$×(1.2+2.3-2.5)
=$\frac{2}{21}$×1
=$\frac{2}{21}$

(5)(1÷2)÷(2÷3)÷(3÷4)÷(5÷6)
=$\frac{1}{2}$$÷\frac{2}{3}$$÷\frac{3}{4}$$÷\frac{5}{6}$
=$\frac{1}{2}$÷($\frac{2}{3}$×$\frac{3}{4}$×$\frac{5}{6}$)
=$\frac{1}{2}$×$\frac{12}{5}$
=$\frac{6}{5}$

点评 考查学生对四则运算法则以及运算定律的掌握情况.

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