Bai 1 : tim x a , 256 + x – 23 = 300 b , 2 . x – 15 = -21 c , 12 – ( 30 – x ) = -223 d , x . ( x – 3 ) = 0 bai 2 : tinh nhanh A = 1 + 2 + 3 + 4

Bạn tham khảo:
Bài 1:
a , $256 + x – 23 = 300 $
$⇔ x = 300 + 23 – 256 $
$⇔ x = 67$

b ,$ 2 . x – 15 = -21$ 
$⇔ 2x = -21 + 15$
$⇔ 2x = -6$
$⇔ x = -6 ÷ 2$
$⇔ x = -3$

c ,$ 12 – ( 30 – x ) = -223$ 
$⇔ 12 – 30 + x = -223 $
$⇔ x = -223 + 30 -12$
$⇔ x = -205$

d , $x . ( x – 3 ) = 0$
⇔ \(\left[ \begin{array}{l}x=0\\x-3=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=0\\x=3\end{array} \right.\)

Bài 2:
$A = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 $
$= ( 1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + 5 $
$= 10 + 10 + 10 + 10 + 5$
$= 45$

$B = 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15$
$= ( 5 + 15 ) + ( 6 + 14 ) + ( 7 + 13 ) + ( 8 + 12 ) + ( 9 + 11 ) + 10$
$= 20 + 20 + 20 + 20 + 20 + 10 $
$= 110$