​b) ( a + b - c ) + ( - b + c ) - ( b + c - a ) - ( a - b - c )
= a + b - c - b + c - b - c + a - a + b + c
= ( a - a ) + ( b - b ) - ( c - c ) - ( b - b ) - ( c - c ) + a
= 0 + 0 - 0 - 0 - 0 + a = a
Vậy ( a + b - c ) + ( - b + c ) - ( b + c - a ) - ( a - b - c ) = a
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