Bài 98. Tìm \(y\), biết:
a) \(- {3 \over 5}.y = {{21} \over {10}}\)
b) \(y:{3 \over 8} = - 1{{31} \over {33}}\)
c) \(1{2 \over 5}.y + {3 \over 7} = - {4 \over 5}\)
d) \( - {{11} \over {12}}.y + 0,25 = {5 \over 6}\)
Hướng dẫn làm bài:
a) \(- {3 \over 5}.y = {{21} \over {10}}\)
\(\Rightarrow y = {{21} \over {10}}:\left( { - {3 \over 5}} \right) = {{21} \over {10}}.\left( { - {5 \over 3}} \right) = - {{21.5} \over {10.3}} = - {7 \over 2} = - 3{1 \over 2}\)
b) \(y:{3 \over 8} = - 1{{31} \over {33}}\)
\( \Leftrightarrow y:{3 \over 8} = - {{64} \over {33}}\)
\(\Rightarrow y = - {{64} \over {33}}.{3 \over 8} = - {{64.3} \over {33.8}} = - {8 \over {11}}\)
c) \(1{2 \over 5}.y + {3 \over 7} = - {4 \over 5}\)
\( \Rightarrow {7 \over 5}y = - {4 \over 5} - {3 \over 7} \Rightarrow {7 \over 5}y = - {{43} \over {35}}\) \(\Rightarrow y = - {{43} \over {35}}:{7 \over 5} \Rightarrow y = - {{43} \over {35}}.{5 \over 7} = - {{43} \over {49}}\)
d) \( - {{11} \over {12}}.y + 0,25 = {5 \over 6}\)
\(\Rightarrow - {{11} \over {12}}y = {5 \over 6} - 0,25 \Rightarrow - {{11} \over {12}}y = {5 \over 6} - {1 \over 4} \)
\(\Rightarrow - {{11} \over {12}}y = {7 \over {12}} \Rightarrow y = {7 \over {12}}:\left( { - {{11} \over {12}}} \right)\)
\(\Rightarrow y = {7 \over {12}}.\left( { - {{12} \over {11}}} \right) \Rightarrow y = - {7 \over {11}}\)