Cho mình hỏi sao x – 3cănx – 4 lại ra (cănx +1) × (cănx – 4) ạ?
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\(\begin{array}{l} x – 3\sqrt x – 4\\ = x – 4\sqrt x + \sqrt x – 4\\ = \sqrt x \left( {\sqrt x – 4} \right) + \left( {\sqrt x – 4} \right)\\ = \left( {\sqrt x + 1} \right)\left( {\sqrt x – 4} \right) \end{array}\)
\(\begin{array}{l}
x – 3\sqrt x – 4\\
= x – 4\sqrt x + \sqrt x – 4\\
= \sqrt x \left( {\sqrt x – 4} \right) + \left( {\sqrt x – 4} \right)\\
= \left( {\sqrt x + 1} \right)\left( {\sqrt x – 4} \right)
\end{array}\)