Bài 3 trang 31 Tài liệu dạy – học Toán 9 tập 1

Đề bài

Tính :

a) $\sqrt 2 \left( {\sqrt {4 – \sqrt 7 }  – \sqrt {4 + \sqrt 7 }  + \sqrt 2 } \right)$;

b) $\left( {4 – \sqrt 7 } \right)\left( {\sqrt 2  + \sqrt {14} } \right)\sqrt {4 + \sqrt 7 }$;

c) $\sqrt {4 + \sqrt {15} }  – \sqrt {4 – \sqrt {15} }  – \sqrt {2 – \sqrt 3 }$;

d) $\sqrt {13 + 30\sqrt {2 + \sqrt {9 + 4\sqrt 2 } } }$.

Lời giải chi tiết

$\begin{array}{l}a)\;\sqrt 2 \left( {\sqrt {4 – \sqrt 7 }  – \sqrt {4 + \sqrt 7 }  + \sqrt 2 } \right)\\ = \sqrt {8 – 2\sqrt 7 }  – \sqrt {8 + 2\sqrt 7 }  + 2\\ = \sqrt {{{\left( {\sqrt 7 } \right)}^2} – 2\sqrt 7  + 1}  – \sqrt {{{\left( {\sqrt 7 } \right)}^2} + 2\sqrt 7  + 1}  + 2\\ = \sqrt {{{\left( {\sqrt 7  – 1} \right)}^2}}  – \sqrt {{{\left( {\sqrt 7  + 1} \right)}^2}}  + 2\\ = \left| {\sqrt 7  – 1} \right| – \left| {\sqrt 7  + 1} \right| + 2\\ = \sqrt 7  – 1 – \sqrt 7  – 1 + 2 = 0\end{array}$

$\begin{array}{l}b)\;\left( {4 – \sqrt 7 } \right)\left( {\sqrt 2  + \sqrt {14} } \right)\sqrt {4 + \sqrt 7 } \\ = \dfrac{1}{2}.2\left( {4 – \sqrt 7 } \right).\sqrt 2 \left( {1 + \sqrt 7 } \right)\sqrt {4 + \sqrt 7 } \\ = \dfrac{1}{2}\left( {8 – 2\sqrt 7 } \right)\left( {1 + \sqrt 7 } \right)\sqrt {8 + 2\sqrt 7 } \\ = \dfrac{1}{2}\left( {{{\left( {\sqrt 7 } \right)}^2} – 2\sqrt 7  + 1} \right)\left( {1 + \sqrt 7 } \right)\sqrt {{{\left( {\sqrt 7 } \right)}^2} + 2\sqrt 7  + 1} \\ = \dfrac{1}{2}{\left( {\sqrt 7  – 1} \right)^2}\left( {1 + \sqrt 7 } \right)\left| {\sqrt 7  + 1} \right|\\ = \dfrac{1}{2}{\left( {\sqrt 7  – 1} \right)^2}\left( {1 + \sqrt 7 } \right)\left( {\sqrt 7  + 1} \right)\\ = \dfrac{1}{2}{\left( {\sqrt 7  – 1} \right)^2}{\left( {\sqrt 7  + 1} \right)^2}\\ = \dfrac{1}{2}{\left[ {\left( {\sqrt 7  – 1} \right)\left( {\sqrt 7  + 1} \right)} \right]^2}\\ = \dfrac{1}{2}{\left( {7 – 1} \right)^2} = \dfrac{1}{2}.36 = 18.\end{array}$

$\begin{array}{l}d)\;\sqrt {13 + 30\sqrt {2 + \sqrt {9 + 4\sqrt 2 } } } \\ = \sqrt {13 + 30\sqrt {2 + \sqrt {{{\left( {2\sqrt 2 } \right)}^2} + 2.2\sqrt 2  + 1} } } \\ = \sqrt {13 + 30\sqrt {2 + \sqrt {{{\left( {2\sqrt 2  + 1} \right)}^2}} } } \\ = \sqrt {13 + 30\sqrt {2 + \left| {2\sqrt 2  + 1} \right|} } \\ = \sqrt {13 + 30\sqrt {2 + 2\sqrt 2  + 1} } \\ = \sqrt {13 + 30\sqrt {{{\left( {\sqrt 2  + 1} \right)}^2}} } \\ = \sqrt {13 + 30\left| {\sqrt 2  + 1} \right|} \\ = \sqrt {13 + 30\left( {\sqrt 2  + 1} \right)} \\ = \sqrt {13 + 30\sqrt 2  + 30} \\ = \sqrt {{{\left( {3\sqrt 2 } \right)}^2} + 2.3\sqrt 2 .5 + {5^2}} \\ = \sqrt {{{\left( {3\sqrt 2  + 5} \right)}^2}}  = \left| {3\sqrt 2  + 5} \right|\\ = 3\sqrt 2  + 5.\end{array}$

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