1 Tính nhanh a) 29,9 . 30,1 b) (31,8) ² – 63,6 .21,8 + (21,8) ² 2 Tìm x a) ( x-3) ² -4 = 0 b) x ² – 2x = 24

Đáp án:

$1.$

$a,  29,9 . 30,1 $

$= (30 – 0,1)(30 + 0 ,1)$

$= 30² – 0,1²$

$= 900 – 0,01$

$= 899,99 $

$b,(31,8)² – 63,6 . 21,8 + (21,8)² $

$= (31,8)² – 2 . 31,8 . 21,8 + (21,8)² $
$= (31,8 – 21,8)² $

$= 10²$

$= 100 $

 $2.$

$a, (x – 3)² – 4 = 0$

$⇔ (x – 3)² – 2² = 0$

$⇔ (x – 3 – 2)(x – 3 + 2) = 0$

$⇔ (x – 5)(x – 1) = 0$

$⇔$ \(\left[ \begin{array}{l}x-5=0\\x-1=0\end{array} \right.\) 

$⇔$ \(\left[ \begin{array}{l}x=5\\x=1\end{array} \right.\) 

$b, x² – 2x = 24 $

$⇔ x² – 2x + 1 – 25 = 0 $

$⇔ (x – 1)² – 5² = 0 $

$⇔ (x – 1 – 5)(x – 1 + 5) = 0 $

$⇔ (x – 6)(x + 4) = 0 $

$⇔$ \(\left[ \begin{array}{l}x-6=0\\x+4=0\end{array} \right.\) 

$⇔$ \(\left[ \begin{array}{l}x=6\\x=-4\end{array} \right.\)