1 . a) - ( 4a + 2b - c ) + ( 3b - 2a + c ) - 5 ( a + b - c )
= - 4a - 2b + c + 3b - 2a + c - 5a - 5b + 5c
= -4a - 2a - 5a - 2b + 3b - 5b + c + c + 5c
= -11a - 4b + 7 c
b) 3 ( a - b - c ) - 4 ( b + c + a ) - 2 ( 3a - 2b - c )
= 3a - 3b - 3c - 4b - 4c - 4a - 6a + 4b + 2c
= 3a - 4a - 6c - 3b - 4b + 4b - 3c - 4c + 2c
= -7a - 3b - 5c
2 . Ta có : ( -17 ) . ( -x ) = / -17 / . / -x / = 17 .x > 0
=> Đpcm