Câu 114 trang 32 Sách BT Toán Lớp 6 tập 2: Tìm x....

Tìm x biết

a) ${\rm{}}0,5{\rm{x}} - {2 \over 3}x = {7 \over {12}}$                   b) $x:4{1 \over 3} =  - 2,5$

c) $5,5{\rm{x}} = {{13} \over {15}}$                               d) ${\rm{}}\left( {{{3{\rm{x}}} \over 7} + 1} \right):\left( { - 4} \right) = {{ - 1} \over {28}}$

Giải

$\eqalign{ & {\rm{a}})0,5{\rm{x}} - {2 \over 3}x = {7 \over {12}} \cr & x.\left( {{1 \over 2} - {2 \over 3}} \right) = {7 \over {12}} \cr & x.\left( {{3 \over 6} - {4 \over 6}} \right) = {7 \over {12}} \cr & x.{{ - 1} \over 6} = {7 \over {12}} \cr & x = {7 \over {12}}:{{ - 1} \over 6} \cr}$

$\eqalign{ & b)x:4{1 \over 3} = - 2,5 \cr & x:{{13} \over 3} = {{ - 5} \over 2} \cr & x = {{ - 5} \over 2}.{{13} \over 3} \cr & x = {{ - 65} \over 6} = - 10{5 \over 6} \cr & x = {7 \over {12}}.{{ - 6} \over 1} = {{ - 7} \over 2} \cr}$

$\eqalign{ & c)5,5{\rm{x}} = {{13} \over {15}} \cr & {{55} \over {10}}x = {{13} \over {15}} \cr & x = {{13} \over {15}}:{{55} \over {10}} \cr & x = {{13} \over {15}}.{{10} \over {55}} \cr & x = {{26} \over {165}} \cr}$

$\eqalign{ & {\rm{d}})\left( {{{3{\rm{x}}} \over 7} + 1} \right):\left( { - 4} \right) = {{ - 1} \over {28}} \cr & {{3{\rm{x}}} \over 7} + 1 = {{ - 1} \over {28}}.\left( { - 4} \right) \cr & {{3{\rm{x}}} \over 7} = {1 \over 7} - 1 \cr & {{3{\rm{x}}} \over 7} = {{ - 6} \over 7} \cr & 3{\rm{x}} = - 3 \cr & x = - 6:3 = - 2 \cr}$