Thực hiện phép tính sau:
a) \({2 \over {3x}} + {5 \over {3x}}\) ;
b) \({5 \over {2ab}} + {9 \over {2ab}}\) ;
c) \({7 \over {6x}} + {4 \over {9y}}\) ;
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d) \({3 \over {2b}} + {1 \over {3b}} + {5 \over 6}\) ;
e) \({{4a} \over {x - 3y}} + {{3a} \over {3x - 9y}}\) .
\(\eqalign{ & a)\,\,{2 \over {3x}} + {5 \over {3x}} = {{2 + 5} \over {3x}} = {7 \over {3x}} \cr & b)\,\,{5 \over {2ab}} + {9 \over {2ab}} = {{5 + 9} \over {2ab}} = {{14} \over {2ab}} = {7 \over {ab}} \cr & c)\,\,{7 \over {6x}} + {4 \over {9y}} = {{7.3y} \over {6x.3y}} + {{4.2x} \over {9y.2x}} = {{21y + 8x} \over {18xy}} \cr & d)\,\,{3 \over {2b}} + {1 \over {3b}} + {5 \over 6} = {{3.3} \over {2b.3}} + {{1.2} \over {3b.2}} + {{5.b} \over {6.b}} = {{9 + 2 + 5b} \over {6b}} = {{11 + 5b} \over {6b}} \cr & e)\,\,{{4a} \over {x - 3y}} + {{3a} \over {3x - 9y}} = {{4a.3} \over {\left( {x - 3y} \right).3}} + {{3a} \over {3\left( {x - 3y} \right)}} = {{12 + 3a} \over {3\left( {x - 3y} \right)}} = {{15a} \over {3\left( {x - 3y} \right)}} = {{5a} \over {x - 3y}} \cr} \)