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

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1. 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 $

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

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

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2. Giải thích các bước giải:

    1.a) `29,9.30,1`

   `=(30-0,1)(30+0,1)`

   `=30^2-0,1^2`

   `=900-0,01=899,99`

    b) `31,8^2-63,6.21,8+21,8^2`

   `=(31,8-21,8)^2`

   `=10^2=100`

    2.a) `(x-3)^2-4=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) `x^2-2x=24`

   `=>(x^2-2x+1)=25`

   `=>(x-1)^2-25=0`

   `=>(x-1-5)(x-1+5)=0`

   `=>(x-6)(x+4)=0`

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

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