Tìm x biết : a) \(x – {1 \over 3} = – {1 \over 2}\) ; b) \( – {2 \over 5} – x = – {3 \over 8}\);
c) \( – {2 \over 3}x – 5 = 7\).
\(\eqalign{ & a) \cr & x – {1 \over 3} = – {1 \over 2} \cr & x = – {1 \over 2} + {1 \over 3} \cr & x = – {3 \over 6} + {2 \over 6} \cr & x = {{ – 1} \over 6} \cr} \) \(\eqalign{ & b) \cr & – {2 \over 5} – x = – {3 \over 8} \cr & – x = {{ – 3} \over 8} + {2 \over 5} \cr & – x = {{ – 15} \over {40}} + {{16} \over {40}} \cr & – x = {1 \over {40}} \cr & x = {{ – 1} \over {40}} \cr} \) \(\eqalign{ & c) \cr & – {2 \over 3}x – 5 = 7 \cr & – {2 \over 3}x = 7 + 5 = 12 \cr & x = 12.{3 \over { – 2}} \cr & x = – 18 \cr} \)