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1. Không dùng máy tính, hãy thực hiện các phép tính sau :
a) \({2 \over 5} – {{ – 1} \over 4}\) ; b) \({{ – 5} \over 6} – {3 \over 4}\);
c) \( – {2 \over 3} – {{ – 1} \over 4}\) ; d) \( – 3 – {1 \over 7}\).
2. Tìm x, biết :
a) \(x + {4 \over 5} = {3 \over 4}\) ;
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b) \({{ – 2} \over 7} – x = {3 \over 8}\) ;
c) \(x – \left( {{{ – 2} \over 3}} \right) = {{ – 1} \over 2}\).
\(\eqalign{ & 1.a){2 \over 5} – {{ – 1} \over 4} = {8 \over {20}} + {5 \over {20}} = {{13} \over {20}} \cr & b){{ – 5} \over 6} – {3 \over 4} = {{ – 10} \over {12}} – {9 \over {12}} = {{ – 19} \over {12}} \cr & c) – {2 \over 3} – {{ – 1} \over 4} = {{ – 8} \over {12}} – {{ – 3} \over {12}} = {{ – 5} \over {12}} \cr & d) – 3 – {1 \over 7} = {{21} \over 7} – {1 \over 7} = {{20} \over 7} \cr & 2.a)x + {4 \over 5} = {3 \over 4} \cr & x = {3 \over 4} – {4 \over 5} \cr & x = {{15} \over {20}} – {{16} \over {20}} \cr & x = {{ – 1} \over {20}} \cr & b){{ – 2} \over 7} – x = {3 \over 8} \cr & – x = {3 \over 8} – {{ – 2} \over 7} \cr & – x = {{21} \over {56}} – {{ – 16} \over {56}} \cr & – x = {{37} \over {56}} \cr & x = – {{37} \over {56}} \cr & c)x – \left( {{{ – 2} \over 3}} \right) = {{ – 1} \over 2} \cr & x = – {1 \over 2} + {{ – 2} \over 3} \cr & x = {{ – 3} \over 6} + {{ – 4} \over 6} \cr & x = {{ – 7} \over 6} \cr} \)