Rút gọn và tính giá trị biểu thức
a, \(\frac{x-11}{\sqrt{x-2}-3}\) x=23-12\(\sqrt{3}\)
b, \(\frac{1}{2\left(1+\sqrt{a}\right)}+\frac{1}{2\left(1-\sqrt{a}\right)}-\frac{a^2+2}{1-a^3}\) với a=\(\sqrt{2}\)
c, \(\sqrt{\frac{\sqrt{a}-1}{\sqrt{b}+1}:\sqrt{\frac{\sqrt{b}-1}{\sqrt{a}+1}}}\) với a=7,25 và b=3,25
d, \(\sqrt{10a^2-4a\sqrt{10}+4}\) với a= \(\sqrt{\frac{2}{5}}+\sqrt{\frac{5}{2}}\)
a/ Với x = \(23-12\sqrt{3}\) ta có:
\(x-11=23-12\sqrt{3}-11=12-12\sqrt{3}=12\left(1-\sqrt{3}\right)\)
\(\sqrt{x-2}-3=\sqrt{23-12\sqrt{3}-2}-3=\sqrt{21-12\sqrt{3}}-3=\sqrt{3^2-2.3.2\sqrt{3}+\left(2\sqrt{3}\right)^2}-3=\sqrt{\left(3-2\sqrt{3}\right)^2}-3=2\sqrt{3}-6\) \(=2\sqrt{3}\left(1-\sqrt{3}\right)\)
=>\(\frac{x-11}{\sqrt{x-2}-3}=\frac{12\left(1-\sqrt{3}\right)}{2\sqrt{3}\left(1-\sqrt{3}\right)}=\frac{12}{2\sqrt{3}}=\frac{2\sqrt{3}.2\sqrt{3}}{2\sqrt{3}}=2\sqrt{3}\)
b/ \(\frac{1}{2\left(1+\sqrt{a}\right)}+\frac{1}{2\left(1-\sqrt{a}\right)}-\frac{a^2+2}{1-a^3}=\frac{1-\sqrt{a}}{2\left(1-a\right)}+\frac{1+\sqrt{a}}{2\left(1-a\right)}-\frac{a^2+2}{\left(1-a\right)\left(1-a+a^2\right)}\)
=\(\frac{2}{2\left(1-a\right)}-\frac{a^2+2}{\left(1-a\right)\left(1-a+a^2\right)}=\frac{1-a+a^2-a^2-2}{\left(1-a\right)\left(1-a+a^2\right)}=\frac{-a-1}{1-a^3}\)
Thay : \(a=\sqrt{2}tacó:\)
\(\frac{-\sqrt{2}-1}{1-\sqrt{2}^3}=\frac{-\left(1+\sqrt{2}\right)}{1-2\sqrt{2}}\)
c/ \(\sqrt{\frac{\sqrt{a}-1}{\sqrt{b}+1}.\frac{\sqrt{\sqrt{b}-1}}{\sqrt{\sqrt{a}+1}}}=\sqrt{\frac{\sqrt{\sqrt{a}-1}}{\sqrt{\sqrt{b}+1}}}\)
Thay a= 7,25; b=3,25 ta có:
\(\sqrt{\frac{\sqrt{\sqrt{7,25}}-1}{\sqrt{\sqrt{3,25}+1}}}\)
d/ \(\sqrt{\left(\sqrt{10}a-2\right)^2}\)
a= \(\sqrt{\frac{2}{5}}+\sqrt{\frac{5}{2}}=\frac{\sqrt{4}+\sqrt{25}}{\sqrt{10}}=\frac{2+5}{\sqrt{10}}=\frac{7}{\sqrt{10}}\)
Thay a=\(\frac{7}{\sqrt{10}}\)ta có:
\(\sqrt{\left(\sqrt{10}.\frac{7}{\sqrt{10}}-2\right)^2}=\sqrt{\left(7-2\right)^2}=5\)
