Bài 27 trang 12 SBT toán 7 tập 1

Đề bài

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) \({\rm{}}\left[ {\left( { - 4,9} \right) + \left( { - 37,8} \right)} \right] + \left[ {\left( { + 1,9} \right) + \left( { + 2,8} \right)} \right]\)

Lời giải

a) \(\left( { - 3,8} \right) + \left[ {\left( { - 5,7} \right) + \left( { + 3,8} \right)} \right]\)

\(= \left( { - 3,8} \right) + {\left( { - 5,7} \right) + \left( { + 3,8} \right)} \)

\(= \left[ {\left( { - 3,8} \right) + \left( { + 3,8} \right)} \right] + \left( { - 5,7} \right)\) 

\(=  0 + (-5,7) = - 5,7\)

b) \(\left( { + 31,4} \right) + \left[ {\left( { + 6,4} \right) + \left( { - 18} \right)} \right]\)

\(=\left( { + 31,4} \right) +  {\left( { + 6,4} \right) + \left( { - 18} \right)} \)

\(= \left[ {\left( { + 31,4} \right) + \left( { - 18} \right)} \right] + \left( { + 6,4} \right)\)  

\(= (+13,4)+(+6,4)=19,8\)

c) \(\left[ {\left( { - 9,6} \right) + \left( { + 4,5} \right)} \right] + \left[ {\left( { + 9,6} \right) + \left( { - 1,5} \right)} \right]\)

\(= {\left( { - 9,6} \right) + \left( { + 4,5} \right)} +{\left( { + 9,6} \right) + \left( { - 1,5} \right)} \)

\(= \left[ {\left( { - 9,6} \right) + \left( { + 9,6} \right)} \right] + \left[ {\left( { + 4,5} \right) + \left( { - 1,5} \right)} \right]\)

\(= 0 + 3 = 3\)

d) \({\rm{}}\left[ {\left( { - 4,9} \right) + \left( { - 37,8} \right)} \right] + \left[ {\left( { + 1,9} \right) + \left( { + 2,8} \right)} \right]\)

\(=  {\left( { - 4,9} \right) + \left( { - 37,8} \right)}  +  {\left( { + 1,9} \right) + \left( { + 2,8} \right)}\)

\(= \left[ {\left( { - 4,9} \right) + \left( { + 1,9} \right)} \right] + \left[ {\left( { - 37,8} \right) + \left( { + 2,8} \right)} \right]\)

\(= \left( { - 3} \right) + \left( { - 35} \right) =  - 38\)


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