Ta có : ( a - b - c ) + ( - a + b - c ) = - ( a - b + c )
<=> a - b - c - a + b + c = - ( a - b + c )
<=> -2c = - ( a - b + c )
<=> -2c - ( -a + b + c ) = 0
<=> -2c + a + b +c = 0
<=> a - b - c = 0
<=> a - ( b + c ) = 0
<=> a = b + c => đpcm
Vậy ....
Hok tốt
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