Câu 20: Đơn giản biểu thức sau khi bỏ dấu ngoặc:
a) (a + b – c) – (b – c + d);
b) –(a – b + c) + (a – b + d);
c) (a + b) – (–a + b – c);
d) –(a + b) + (a + b + c);
e) (a – b + c) – (a – b + c);
f) –(a – b – c) + (a – b – c).
a) (a + b – c) – (b – c + d)
= a + b – c – b + c – d
= a + (b – b) + (–c + c) – d
= a – d.
b) –(a – b + c) + (a – b + d)
= –a + b – c + a – b + d
= (a – a) + (b – b) – c + d
= d – c.
c) (a + b) – (–a + b – c)
= a + b + a – b + c
= (a + a) + (b – b) + c
= 2a + c.
d) –(a + b) + (a + b + c)
= –a – b + a + b + c
= (–a + a) + (–b + b) + c
= c.
e) (a – b + c) – (a – b + c)
= a – b + c – a + b – c
= (a – a) + (–b + b) + (c – c)
= 0.
f) –(a – b – c) + (a – b – c)
= –a + b + c + a – b – c
= (–a + a) + (b – b) + (c – c)
= 0.