a/ A = ( -a + b - c ) - ( -a - b - c )
= -a + b - c + a + b + c
= ( a - a ) + ( b + b ) + ( c - c )
= 0 + 2b + 0
= 2b.
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b/ Với a = 1; b = -1; c = -2, có:
A = ( -a + b - c ) - ( -a - b - c )
= [ -1 + (-1) - (-2) ] - [ -1 - (-1) - (-2) ]
= [ -2 + 2 ] - [ 0 + 2 ]
= 0 - 2
= -2.