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