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\(\sqrt{46-6\sqrt{5}}-\sqrt{29-12\sqrt{5}}\\ =\sqrt{\left(3\sqrt{5}\right)^2-2.3\sqrt{5}+1}-\sqrt{\left(2\sqrt{5}\right)^2-2.3.2\sqrt{5}+3^2}\\ =3\sqrt{5}-1-2\sqrt{5}+3=\sqrt{5}+2\)
Mấy câu sau tương tự.
a. Không giải được\(\sqrt{29}-6\sqrt{6}< 0\)
b. \(\left(\sqrt{8}-3\sqrt{2}-\sqrt{10}\right)\cdot\sqrt{2}-\sqrt{20}\)
=\(\left(2\sqrt{2}-3\sqrt{2}-\sqrt{10}\right)\cdot\sqrt{2}-\sqrt{20}\)
=\(\left(\sqrt{2}-\sqrt{10}\right)\cdot\sqrt{2}-\sqrt{20}\)
a) Không thể giải vì \(\sqrt{29}-6\sqrt{6}< 0\)
b) \(\left(\sqrt{8}-3\sqrt{2}-\sqrt{10}\right)\cdot\sqrt{2}-\sqrt{20}\)
=\(\left(2\sqrt{2}-3\sqrt{2}-\sqrt{10}\right)\cdot\sqrt{2}-\sqrt{20}\)
=\(\left(-\sqrt{2}-\sqrt{10}\right)\cdot\sqrt{2}-\sqrt{20}\)
=\(-2-2\sqrt{5}-2\sqrt{5}\)
=\(-2-4\sqrt{5}\)
=\(-2\left(1+2\sqrt{5}\right)\)
\(\sqrt{242}.\sqrt{26}.\sqrt{130}.\sqrt{0,9}-\left(\sqrt{2}-1\right)\left(\sqrt{2}+1\right)\)
\(=\sqrt{121}.\sqrt{2}.\sqrt{2}.\sqrt{13}.\sqrt{13}.\sqrt{10}.\sqrt{0,9}-\left(2-1\right)\)
\(=11.2.13.\sqrt{9}-1=286.3-1=857\)
\(\frac{3-\sqrt{6}}{\sqrt{12}-\sqrt{8}}-\frac{\sqrt{15}-\sqrt{5}}{2\sqrt{12}-4}+\frac{\sqrt{17-4\sqrt{15}}}{4}\)
\(=\frac{\sqrt{3}\left(\sqrt{3}-\sqrt{2}\right)}{2\left(\sqrt{3}-\sqrt{2}\right)}-\frac{\sqrt{5}\left(\sqrt{3}-1\right)}{4\left(\sqrt{3}-1\right)}+\frac{\sqrt{\left(2\sqrt{3}-\sqrt{5}\right)^2}}{4}\)
\(=\frac{\sqrt{3}}{2}-\frac{\sqrt{5}}{4}+\frac{2\sqrt{3}-\sqrt{5}}{4}\)
\(=\sqrt{3}-\frac{\sqrt{5}}{4}\)
\(\sqrt{49-5\sqrt{96}}+\sqrt{49+5\sqrt{96}}=\sqrt{25-2\cdot5\cdot2\sqrt{6}+24}+\sqrt{25-2\cdot5\cdot2\sqrt{6}+24}=\sqrt{\left(5+2\sqrt{6}\right)^2}+\sqrt{\left(5-2\sqrt{6}\right)^2}=5+2\sqrt{6}+5-2\sqrt{6}=10\) ---
\(\sqrt{13-\sqrt{160}}+\sqrt{53+4\sqrt{90}}=\sqrt{8-2\sqrt{5}\cdot\sqrt{8}+5}+\sqrt{45+2\cdot3\sqrt{5}\cdot\sqrt{8}+8}=\sqrt{\left(\sqrt{8}-\sqrt{5}\right)^2}+\sqrt{\left(3\sqrt{5}+\sqrt{8}\right)^2}=\sqrt{8}-\sqrt{5}+3\sqrt{5}+\sqrt{8}=2\sqrt{8}+2\sqrt{5}\)
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\(\sqrt{11-6\sqrt{2}}+\sqrt{3-2\sqrt{2}}=\sqrt{9-2\cdot3\cdot\sqrt{2}+2}+\sqrt{2-2\sqrt{2}+1}=\sqrt{\left(3-\sqrt{2}\right)^2}+\sqrt{\left(\sqrt{2}-1\right)^2}=3-\sqrt{2}+\sqrt{2}-1=2\)
---
\(\sqrt{15-6\sqrt{6}}+\sqrt{35-12\sqrt{6}}=\sqrt{9-2\cdot3\cdot\sqrt{6}+6}+\sqrt{27-2\cdot\sqrt{27}\cdot\sqrt{8}+8}=\sqrt{\left(3-\sqrt{6}\right)^2}+\sqrt{\left(3\sqrt{3}-2\sqrt{2}\right)^2}=3-\sqrt{6}+3\sqrt{3}-2\sqrt{2}\)
---
\(\sqrt{17-3\sqrt{32}}+\sqrt{17+3\sqrt{32}}=\sqrt{9-2\cdot3\cdot2\sqrt{2}+8}+\sqrt{9+2\cdot2\cdot2\sqrt{2}+8}=\sqrt{\left(3-2\sqrt{2}\right)^2}+\sqrt{\left(3+2\sqrt{2}\right)^2}=3-2\sqrt{2}+3+2\sqrt{2}=6\)
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a) \(2^2=4\)
\(\sqrt{3^2}=3\)
\(4>3\Rightarrow\) \(2>\sqrt{3}\)
b) \(6^2=36\)
\(\sqrt{41^2}=41\)
\(36< 41\Rightarrow6< \sqrt{41}\)
b)\(\sqrt{17-12\sqrt{2}}\)
=\(\sqrt{9-2.3.2\sqrt{2}+8}\)
=\(\sqrt{\left(3-2\sqrt{2}\right)^2}\)
= \(3-2\sqrt{2}\)
Câu 1. Biến đổi biểu thức trong căn thành một bình phương một tổng hay một hiệu rồi từ đó phá bớt một lớp căn
a/\(\sqrt{41+12\sqrt{5}}\)
Ta có \(a^2=15+2.\sqrt{15.14}+14=29+2.\sqrt{210}\)
\(b^2=17+2.\sqrt{17.12}+12=29+2.\sqrt{204}\)
Dễ thấy \(a^2>b^2\)mà a,b>0 suy ra a>b