Cộng các phân số sau:
\(a)\,\,{{ - 2} \over 3} + {4 \over {15}};\,\,\,\,\,\,\,\,b)\,\,{{11} \over {15}} + {9 \over { - 10}};\,\,\,\,\,c)\,\,{1 \over { - 7}} + 3\)
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\(\eqalign{& a)\,\,{{ - 2} \over 3} + {4 \over {15}} = {{ - 2.5} \over {3.5}} + {4 \over {15}} = {{ - 10} \over {15}} + {4 \over {15}} = {{ - 10 + 4} \over {15}} = {{ - 6} \over {15}};\,\,\,\,\,\,\,\, \cr & b)\,\,{{11} \over {15}} + {9 \over { - 10}} = {{11.2} \over {15.2}} + {{9.( - 3)} \over { - 10.( - 3)}} = {{22} \over {30}} + {{ - 27} \over {30}} = {{22 + ( - 27)} \over {30}} = {{ - 5} \over {30}};\,\,\,\,\, \cr & c)\,\,{1 \over { - 7}} + 3 = {1 \over { - 7}} + {{ - 21} \over { - 7}} = {{1 + ( - 21)} \over { - 7}} = {{ - 20} \over { - 7}} = {{20} \over 7} \cr} \)