1 phân tích thành nhân tử a,x ² – 9 b,4x ² – 25 c,6x-9-x ² 2 tìm x a, x+5x ²=0 b,x+1=(x+1) ² c, x ³+x=0 3 tính nhanh a, 85.127+5.3.127 b,14.46+86.46

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1. $\text{Bài 1:}$

   $a)x²-9=(x-3)(x+3)$

   $b)4x²-25=(2x-5)(2x+5)$

   $\text{c) 6x-9-x²}$

   $=-x²+6x-9$

   $=-(x²-6x+9)$

   $=-(x-3)²$

   $\text{Bài 2}$

   $\text{a) x+5x²=0}$

   $(=)x(1+5x)=0$

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

   $\text{S={0;$\frac{-1}{5}$}}$

   $\text{b)x+1=(x+1)²}$

   $(=)x+1-(x+1)²=0$

   $(=)(x+1)(1-x-1)=0$

   $(=)(x+1)x=0$

   $(=)$\(\left[ \begin{array}{l}x+1=0\\2-x\end{array} \right.\) (=)\(\left[ \begin{array}{l}x=-1\\x=0\end{array} \right. \)

   $S={-1;0}$

   $\text{c) x ³+x=0}$

   $(=)x(x²+1)=0$

   $(=)$\(\left[ \begin{array}{l}x²+1=0\\x=0\end{array} \right.\) (=)\(\left[ \begin{array}{l}x²=-1=) \text{vô nghiệm}\\x=0\end{array} \right. \)

   $S={0}$

   $\text{Bài 3}$

   $\text{a) 85.127+5.3.127}$

   $=85.127+15.127$

   $=127.(15+85)$

   $=127.100$

   $=12700$

   $\text{b)14.46+86.46}$

   $=46.(14+86)$

   $=46.100$

   $=4600$

   $\text{c) 86.153-530.8,6}$

   $=86.153-530.86:10$

   $=86.153-86.53$

   $=86(153-53)$

   $=86.100$

   $=8600$

   $\text{d) 97.13+130.0,3}$

   $=97.13+13.10.3:10$

   $=97.13+13.3$

   $=3.(97+3)$

   $=3.100$

   $=300$

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2. Bài 1:

   a,x²-9=x²-3²=(x-3).(x+3)

   b,4x²-25=2².x²-5²=(2x)²-5²=(2x-5).(2x+5)

   c,6x-9-x²=-(x²-6x+9)=-(x-3)²

   Bài 2:

   a,x+5x²=0

   ⇒x(1+5x)=0

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

   vậy x=0 và x=-1/5

   b,x+1=(x+1)²

   ⇒(x+1)-(x+1)²=0

   ⇒(x+1)(1-x-1)=0

   ⇒-x(x+1)=0

   ⇒\(\left[ \begin{array}{l}-x=0\\x+1=0\end{array} \right.\) 

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

   vậy x=0 và x=-1

   c,x³+x=0

   ⇒x(x²+1)=0

   ⇒\(\left[ \begin{array}{l}x=0\\x²+1=0\end{array} \right.\) 

   ⇒\(\left[ \begin{array}{l}x=0\\x∈∅\end{array} \right.\) 

   ⇒x=0

   vậy x=0

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