Câu 71. Tìm x, biết:

a) \(x-\frac{1}{4}=\frac{5}{8}.\frac{2}{3}\) ;                       b) \(\frac{x}{126}=\frac{-5}{9}.\frac{4}{7}\) ;

Hướng dẫn giải.

a) 

\(\eqalign{
& x - {1 \over 4} = {5 \over 8}.{2 \over 3} \cr
& x - {1 \over 4} = {5 \over {12}} \cr
& x\,\,\,\,\,\,\,\,\,\, = {5 \over {12}} + {1 \over 4} \cr
& x\,\,\,\,\,\,\,\,\,\, = {8 \over {12}} = {2 \over 3} \cr} \)

b) 

\(\eqalign{
& {x \over {126}} = {{ - 5} \over 9}.{4 \over 7} \cr
& {x \over {126}} = {{ - 20} \over {63}} \cr
& 63.x = - 20.126 \cr
& \,\,\,\,\,\,x = {{ - 20.126} \over {63}} \cr
& \,\,\,\,\,\,x = - 40 \cr} \)