Chứng tỏ (a – b + c) – (a + c) = -b

18. Chứng tỏ
1/ (a – b + c) – (a + c) = -b
2/ (a + b) – (b – a) + c = 2a + c
3/ - (a + b – c) + (a – b – c) = -2b
4/ a(b + c) – a(b + d) = a(c – d) 5/ a(b – c) + a(d + c) = a(b + d)