Toán Lớp 8: 1) 3x^2-12x-15=0
2) x^2-4x-12=0
3) x^2-2x+7=0
4) 2x^3-50x=0 (giúp em với , em hứa sẽ vote 5 sao ạ )
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Toán Lớp 8: 1) 3x^2-12x-15=0
2) x^2-4x-12=0
3) x^2-2x+7=0
4) 2x^3-50x=0 (giúp em với , em hứa sẽ vote 5 sao ạ )
Comments ( 2 )
a)3{x^2} – 12x – 15 = 0\\
\Rightarrow 3\;\left( {{x^2} – 4x – {\rm{ }}5} \right)\; = 0\\
\Rightarrow 3\left( {{x^2}\; – {\rm{ }}5x\; + \;1x – 5} \right) = 0\\
\Rightarrow 3\left( {x + 1} \right)\left( {x – 5} \right) = 0\\
\Rightarrow \left[ \begin{array}{l}
3 = 0\,\left( \emptyset \right)\\
x + 1 = 0\\
x – 5 = 0
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
3 = 0\,\left( \emptyset \right)\\
x = – 1\\
x = 5
\end{array} \right.\\
b){x^2} – 4x – 12 = 0\\
\Rightarrow \;{x^2} – 6x + 2x – 12 = 0\\
\Rightarrow x(x – 6) + 2(x – 6) = 0\;\\
\Rightarrow (x – 6)(x + 2) = 0\;\\
\Rightarrow \left[ \begin{array}{l}
x – 6 = 0\\
x + 2 = 0
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
x = 6\\
x = – 2
\end{array} \right.\\
c)\;{x^2} – 2x + 7 = 0\;\\
\Rightarrow {x^2} – 2x + 1 + 6 = 0\;\\
\Rightarrow {(x – 1)^2} = – 6\left( \emptyset \right)\;\\
{(x – 1)^2} \ge 0\;\forall x\\
d)\;2{x^3} – 50x = 0\;\\
\Rightarrow 2x({x^2} – 25) = 0\;\\
\Rightarrow 2x(x – 5)(x + 5) = 0\;\\
\Rightarrow \left[ \begin{array}{l}
2x = 0\\
x – 5 = 0\\
x + 5 = 0
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
x = 0\\
x = – 5\\
x = 5
\end{array} \right.
\end{array}\]