Question

16．求未知數(shù)x．
①$\frac{0.75}{x}$=$\frac{25}{8}$
②x-$\frac{2}{9}$x=42．

Answer 1

分析 ①把$\frac{0.75}{x}$=$\frac{25}{8}$看作0.75：x=25：8根據(jù)比例性質(zhì)，把原式轉(zhuǎn)化為25x=0.75×8，根據(jù)等式性質(zhì)，方程兩邊同除以25求解． 
②先運用乘法分配律的運算改寫成（1-$\frac{2}{9}$）x=42，即得$\frac{7}{9}$x=42，再根據(jù)等式性質(zhì)，兩邊同乘以$\frac{9}{7}$即可．

解答 解：①$\frac{0.75}{x}$=$\frac{25}{8}$                                              
             25x=0.75×8                                                
  （25÷25）x=0.75×8÷25                              
                 x=0.24                                                        
②x-$\frac{2}{9}$x=42
（1-$\frac{2}{9}$）x=42
       $\frac{7}{9}$x=42
  $\frac{7}{9}$x×$\frac{9}{7}$=42×$\frac{9}{7}$
         x=54

點評 ①本題主要考查比例性質(zhì)和等式性質(zhì)解方程能力，注意等號對齊．
②此題考查了根據(jù)等式性質(zhì)解方程，即等式兩邊同加、同減、同乘、同除（0除外）等式左右兩邊仍相等，注意等號上下對齊．