Advertisements (Quảng cáo)
Tính các tổng (hiệu) dưới đây sau khi đã rút gọn phân số :
a) \({9 \over {36}} + {7 \over { – 21}}\) ; b) \({{ – 5} \over {35}} + {6 \over {42}}\) ;
c) \({{ – 18} \over {24}} – {{15} \over {21}}\) ; d) \({{ – 15} \over {18}} – {{21} \over {35}}\) ;
e) \({4 \over { – 20}} + {{14} \over { – 21}}\) ; f) \({{18} \over { – 30}} – {8 \over {28}}\).
Advertisements (Quảng cáo)
\(\eqalign{ & a){9 \over {36}} + {7 \over { – 21}} = {1 \over 4} + {{ – 1} \over 3} = {3 \over {12}} + {{ – 4} \over {12}} = {{3 – 4} \over {12}} = {{ – 1} \over {12}} \cr & b){{ – 5} \over {35}} + {6 \over {42}} = {{ – 1} \over 7} + {1 \over 7} = {{ – 1 + 1} \over 7} = {0 \over 7} = 0 \cr & c){{ – 18} \over {24}} – {{15} \over {21}} = {{ – 3} \over 4} – {5 \over 7} = {{ – 21} \over {28}} – {{20} \over {28}} = {{ – 41} \over {28}} \cr & d){{ – 15} \over {18}} – {{ – 21} \over {35}} = {{ – 5} \over 6} – {{ – 3} \over 5} = {{ – 25} \over {30}} – {{ – 18} \over {30}} = {{ – 7} \over {30}} \cr & e){4 \over { – 20}} + {{14} \over { – 21}} = {{ – 1} \over 5} + {{ – 2} \over 3} = {{ – 3} \over {15}} + {{ – 10} \over {15}} = {{ – 13} \over {15}} \cr & h){{18} \over { – 30}} – {8 \over {28}} = {{ – 3} \over 5} – {2 \over 7} = {{ – 21} \over {35}} – {{10} \over {35}} = {{ – 31} \over {35}} \cr} \)