Toán phân tích x^2- √x+x-1 thành nhân tử so sánh √30 – √ 29 và √29 – √28 14/08/2021 By Amara phân tích x^2- √x+x-1 thành nhân tử so sánh √30 – √ 29 và √29 – √28
Đáp án: $\begin{array}{l}A = {x^2} – \sqrt x + x – 1\\ = \sqrt x .\left( {x\sqrt x – 1} \right) + \left( {x – 1} \right)\\ = \sqrt x .\left( {\sqrt x – 1} \right)\left( {x + \sqrt x + 1} \right) + \left( {\sqrt x – 1} \right)\left( {\sqrt x + 1} \right)\\ = \left( {\sqrt x – 1} \right)\left( {\sqrt x .\left( {x + \sqrt x + 1} \right) + \sqrt x + 1} \right)\\ = \left( {\sqrt x – 1} \right)\left( {x\sqrt x + x + 2\sqrt x + 1} \right)\\b)B = \sqrt {30} – \sqrt {29} \\ = \dfrac{{\left( {\sqrt {30} + \sqrt {29} } \right)\left( {\sqrt {30} – \sqrt {29} } \right)}}{{\sqrt {30} + \sqrt {29} }}\\ = \dfrac{{30 – 29}}{{\sqrt {30} + \sqrt {29} }} = \dfrac{1}{{\sqrt {30} + \sqrt {29} }}\\C = \sqrt {29} – \sqrt {28} \\ = \dfrac{{29 – 28}}{{\sqrt {29} + \sqrt {28} }} = \dfrac{1}{{\sqrt {29} + \sqrt {28} }}\\Do:\sqrt {30} + \sqrt {29} > \sqrt {29} + \sqrt {28} \\ \Rightarrow \dfrac{1}{{\sqrt {30} + \sqrt {29} }} < \dfrac{1}{{\sqrt {29} + \sqrt {28} }}\\ \Rightarrow B < C\\Hay\,\sqrt {30} – \sqrt {29} < \sqrt {29} – \sqrt {28} \end{array}$ Trả lời
Đáp án:
$\begin{array}{l}
A = {x^2} – \sqrt x + x – 1\\
= \sqrt x .\left( {x\sqrt x – 1} \right) + \left( {x – 1} \right)\\
= \sqrt x .\left( {\sqrt x – 1} \right)\left( {x + \sqrt x + 1} \right) + \left( {\sqrt x – 1} \right)\left( {\sqrt x + 1} \right)\\
= \left( {\sqrt x – 1} \right)\left( {\sqrt x .\left( {x + \sqrt x + 1} \right) + \sqrt x + 1} \right)\\
= \left( {\sqrt x – 1} \right)\left( {x\sqrt x + x + 2\sqrt x + 1} \right)\\
b)B = \sqrt {30} – \sqrt {29} \\
= \dfrac{{\left( {\sqrt {30} + \sqrt {29} } \right)\left( {\sqrt {30} – \sqrt {29} } \right)}}{{\sqrt {30} + \sqrt {29} }}\\
= \dfrac{{30 – 29}}{{\sqrt {30} + \sqrt {29} }} = \dfrac{1}{{\sqrt {30} + \sqrt {29} }}\\
C = \sqrt {29} – \sqrt {28} \\
= \dfrac{{29 – 28}}{{\sqrt {29} + \sqrt {28} }} = \dfrac{1}{{\sqrt {29} + \sqrt {28} }}\\
Do:\sqrt {30} + \sqrt {29} > \sqrt {29} + \sqrt {28} \\
\Rightarrow \dfrac{1}{{\sqrt {30} + \sqrt {29} }} < \dfrac{1}{{\sqrt {29} + \sqrt {28} }}\\
\Rightarrow B < C\\
Hay\,\sqrt {30} – \sqrt {29} < \sqrt {29} – \sqrt {28}
\end{array}$