Tính bằng cách thuận tiện nhất :
a)
(976 + 865) + 135 = ...
891 + (799 + 109) = ...
b)
\(\displaystyle\left( {{2 \over 5} + {7 \over 9}} \right) + {3 \over 5} =\,... \\\displaystyle{{19} \over {11}} + \left( {{8 \over {13}} + {3 \over {11}}} \right)=\,...\)
c)
16,88 + 9,76 + 3,12 = ...
72,84 + 17,16 + 82,84 = ...
a)
\(\displaystyle\left( {976 + 865} \right) + 135 = 976 + \left( {865 + 135} \right)\displaystyle= 976 + 1000 = 1976 \\\displaystyle891 + \left( {799 + 109} \right) = \left( {891 + 109} \right) + 799 \displaystyle= 1000 + 799 = 1799\)
b)
\(\displaystyle\left( {{2 \over 5} + {7 \over 9}} \right) + {3 \over 5} = \left( {{2 \over 5} + {3 \over 5}} \right) + {7 \over 9} \displaystyle = \dfrac{5}{5} + {7 \over 9} = 1 + {7 \over 9} = 1{7 \over 9} \\\displaystyle{{19} \over {11}} + \left( {{8 \over {13}} + {3 \over {11}}} \right) \displaystyle= \left( {{{19} \over {11}} + {3 \over {11}}} \right) + {8 \over {13}}\displaystyle= {{22} \over {11}} +{8 \over {13}}\displaystyle= 2 + {8 \over {13}} = 2{8 \over {13}}\)
c)
16,88 + 9,76 + 3,12 = (16,88 + 3,12) + 9,76 = 20 + 9,76 = 29,76
72,84 + 17,16 + 82,84 = 72,84 + (17,16 + 82,84) = 72,84 + 100 = 172,84
-- Mod Toán lớp 5