Bạn tham khảo:
1, $( a + b + c ) - ( a - b - c )$
$= a + b + c - a + b + c$
$= ( a - a ) + ( b + b ) + ( c + c )$
$= 2b + 2c$
$= 2( b + c )$
2, $( a - b ) - ( a - c ) + ( a + b ) - c$
$= a - b - a + c + a + b - c$
$= ( a - a + a ) + ( -b + b ) + ( c - c )$
$= a + 0 + 0$
$= a$