Tìm ba cặp số có tổng bằng nhau trong mỗi câu dưới đây:
a) -7; 4; -2; -11; 2; 8; -5
b) 47; -25; 41; 26; -19; -27; 49.
c) \( – {2 \over 5};\,\,{1 \over 5};\,\,{2 \over 5};\,\, – {1 \over 3};\,\,{4 \over 5};\,\,{{ – 1} \over 5};\,\,1\)
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\(\eqalign{ & a) \bullet ( – 11) + 4 = ( – 2) + ( – 5) \cr & \bullet ( – 7) + 4 = ( – 11) + 8 \cr & \bullet 4 + 2 = ( – 2) + 8 \cr & b) \bullet 47 + ( – 25) = 41 + ( – 19) \cr & \bullet 41 + ( – 19) = 49 + ( – 27) \cr & \bullet 47 + ( – 25) = 49 + ( – 27) \cr & c) \bullet \left( { – {2 \over 5}} \right) + {2 \over 5} = {1 \over 5} + \left( {{{ – 1} \over 5}} \right) \cr & \bullet \left( { – {2 \over 5}} \right) + 1 = {1 \over 5} + {2 \over 5} \cr & \bullet {1 \over 5} + 1 = {2 \over 5} + {4 \over 5} \cr} \)