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Tính bằng cách hợp lý giá trị của các biểu thức sau:
a) \(\left( { – 3,8} \right) + \left[ {\left( { – 5,7} \right) + \left( { + 3,8} \right)} \right]\)
b) \(\left( { + 31,4} \right) + \left[ {\left( { + 6,4} \right) + \left( { – 18} \right)} \right]\)
c) \(\left[ {\left( { – 9,6} \right) + \left( { + 4,5} \right)} \right] + \left[ {\left( { + 9,6} \right) + \left( { – 1,5} \right)} \right]\)
d) \({\rm{}}\left[ {\left( { – 4,9} \right) + \left( { – 37,8} \right)} \right] + \left[ {\left( { + 1,9} \right) + \left( { + 2,8} \right)} \right]\)
a) \(\left( { – 3,8} \right) + \left[ {\left( { – 5,7} \right) + \left( { + 3,8} \right)} \right]\)
= \(\left[ {\left( { – 3,8} \right) + \left( { + 3,8} \right)} \right] + \left( { – 5,7} \right)\)
= 0 + (-5,7) = – 5,7
b) \(\left( { + 31,4} \right) + \left[ {\left( { + 6,4} \right) + \left( { – 18} \right)} \right]\)
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= \(\left[ {\left( { + 31,4} \right) + \left( { – 18} \right)} \right] + \left( { + 6,4} \right)\)
= (+31,4)+(+6,4)=19,8
c) \(\left[ {\left( { – 9,6} \right) + \left( { + 4,5} \right)} \right] + \left[ {\left( { + 9,6} \right) + \left( { – 1,5} \right)} \right]\)
= \(\left[ {\left( { – 9,6} \right) + \left( { + 9,6} \right)} \right] + \left[ {\left( { + 4,5} \right) + \left( { – 1,5} \right)} \right]\)
= 0 + 3 = 3
d) \({\rm{}}\left[ {\left( { – 4,9} \right) + \left( { – 37,8} \right)} \right] + \left[ {\left( { + 1,9} \right) + \left( { + 2,8} \right)} \right]\)
= \(\left[ {\left( { – 4,9} \right) + \left( { + 1,9} \right)} \right] + \left[ {\left( { – 37,8} \right) + \left( { + 2,8} \right)} \right]\)
= \(\left( { – 3} \right) + \left( { – 35} \right) = – 38\)