Bài 98 trang 49 SGK Toán 7 tập 1

Đề bài

Tìm \(y\), biết:

\(\eqalign{
& a)\,\, - {3 \over 5}.y = {{21} \over {10}} \cr
& b)\,\,y:{3 \over 8} = - 1{{31} \over {33}} \cr
& c)\,\,1{2 \over 5}.y + {3 \over 7} = - {4 \over 5} \cr
& d)\,\, - {{11} \over {12}}.y + 0,25 = {5 \over 6} \cr} \)

Lời giải chi tiết

\(\eqalign{
& a)\,\, - {3 \over 5}.y = {{21} \over {10}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,y = {{21} \over {10}}:\left( { - {3 \over 5}} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,y = {{21} \over {10}}.\left( { - {5 \over 3}} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,y = - {{21.5} \over {10.3}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,y = - {7 \over 2} \cr
& b)\,\,y:{3 \over 8} = - 1{{31} \over {33}} \cr
& \,\,\,\,\,\,\,\,y:{3 \over 8} = - {{64} \over {33}} \cr
& \,\,\,\,\,\,\,\,y = \left( { - {{64} \over {33}}} \right).{3 \over 8} \cr
& \,\,\,\,\,\,\,\,y = - {{64.3} \over {33.8}} \cr
& \,\,\,\,\,\,\,\,y = - {8 \over {11}} \cr
& c)\,\,1{2 \over 5}.y + {3 \over 7} = - {4 \over 5} \cr
& \,\,\,\,\,\,\,\,\,{7 \over 5}y = - {4 \over 5} - {3 \over 7} \cr
& \,\,\,\,\,\,\,\,{7 \over 5}y = - {{28} \over {35}} - {{15} \over {35}} \cr
& \,\,\,\,\,\,\,\,{7 \over 5}y = {{ - 43} \over {35}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,y = {{ - 43} \over {35}}:{7 \over 5} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,y = {{ - 43} \over {35}}.{5 \over 7} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,y = {{ - 43} \over {49}} \cr
& d)\,\, - {{11} \over {12}}.y + 0,25 = {5 \over 6} \cr
& \,\,\,\,\,\,\,\, - {{11} \over {12}}y = {5 \over 6} - 0,25 \cr
& \,\,\,\,\,\,\, - {{11} \over {12}}y = {5 \over 6} - {{25} \over {100}} \cr
& \,\,\,\,\,\,\,\,\, - {{11} \over {12}}y = {5 \over 6} - {1 \over 4} \cr
& \,\,\,\,\,\,\,\, - {{11} \over {12}}y = {{10} \over {12}} - {3 \over {12}} \cr
& \,\,\,\,\,\,\,\,\, - {{11} \over {12}}y = {7 \over {12}} \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,y = {7 \over {12}}:\left( { - {{11} \over {12}}} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,y = {7 \over {12}}.\left( { - {{12} \over {11}}} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,y = - {7 \over {11}} \cr} \)

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