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