Câu 89: Tính tổng :
a) (-24) + 6 + 10 + 24 b) 15 +23+( -25) +(-23)
c) (-3) + ( -350 ) + (-7) + 350 d) ( -9 ) + ( -11 ) + 21 + ( -1)
a) \(\left( { – 24} \right) + 6 + 10 + 24\)
= \(\left[ {\left( { – 24} \right) + 24} \right] + \left( {6 + 10} \right) \)
= 0 + 16 = 16
b) 15 +23+( -25) +(-23)
= \(\left[ {15 + \left( { – 25} \right)} \right] + \left[ {23 + \left( { – 23} \right)} \right] = \left( { – 10} \right) + 0 = – 10\)
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c) (-3) + ( -350 ) + (-7) +350
= \(\left[ {\left( { – 3} \right) + \left( { – 7} \right)} \right] + \left[ {\left( { – 350} \right) + 350} \right] = \left( { – 10} \right) + 0 = – 10\)
d) ( -9 ) + ( -11 ) +21 + ( -1 )= 21 – (9+11+1) = 21 – 21 =0
Câu 90: Đơn giản biểu thức:
a) x + 25 + ( -17 ) + 63 b) ( -75) – (p + 20 ) + 95
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a) \(x + {\rm{ }}25{\rm{ }} + {\rm{ }}\left( {{\rm{ }} – 17{\rm{ }}} \right){\rm{ }} + {\rm{ }}63 = x + \left[ {\left( {25 + 63} \right) + \left( { – 17} \right)} \right]\)
= \(x + \left[ {88 + \left( { – 17} \right)} \right] = x + 71\)
b) ( -75) – (p + 20 ) + 95 = (-75) – p – 20 + 95
= -p +( 95 – 75 – 20 ) = – p + 0 = -p
Câu 91: Tính nhanh các tổng sau:
a) (5674 – 97) – 5674 b) (-1075 ) – ( 29 – 1075 )
a) (5674 – 97) – 5674 = 5674 – 97 – 5674
= (5674 – 5674) – 97 = 0 – 97 = -97
b) (-1075) – (29 – 1075 ) = ( -1075 ) – 29 + 1075
= (- 1075 + 1075 ) – 29 = 0 – 29 = -29
