Toán Lớp 9: a,13/2√3-5
b8/√5-3
c,1/5√2-2√5
d,√5-√7/√5+√7
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Toán Lớp 9: a,13/2√3-5
b8/√5-3
c,1/5√2-2√5
d,√5-√7/√5+√7
Comments ( 2 )
a) – 2\sqrt 3 – 5\\
b) – 2\sqrt 5 – 6\\
c)\dfrac{{5\sqrt 2 + 2\sqrt 5 }}{{30}}\\
d) – 6 + \sqrt {35}
\end{array}\)
a)\dfrac{{13}}{{2\sqrt 3 – 5}} = \dfrac{{13\left( {2\sqrt 3 + 5} \right)}}{{4.3 – 25}}\\
= – \dfrac{{13\left( {2\sqrt 3 + 5} \right)}}{{13}}\\
= – \left( {2\sqrt 3 + 5} \right)\\
= – 2\sqrt 3 – 5\\
b)\dfrac{8}{{\sqrt 5 – 3}} = \dfrac{{8\left( {\sqrt 5 + 3} \right)}}{{5 – 9}}\\
= – \dfrac{{8\left( {\sqrt 5 + 3} \right)}}{4}\\
= – 2\left( {\sqrt 5 + 3} \right)\\
= – 2\sqrt 5 – 6\\
c)\dfrac{1}{{5\sqrt 2 – 2\sqrt 5 }} = \dfrac{{5\sqrt 2 + 2\sqrt 5 }}{{25.2 – 4.5}}\\
= \dfrac{{5\sqrt 2 + 2\sqrt 5 }}{{30}}\\
d)\dfrac{{\sqrt 5 – \sqrt 7 }}{{\sqrt 5 + \sqrt 7 }} = \dfrac{{{{\left( {\sqrt 5 – \sqrt 7 } \right)}^2}}}{{5 – 7}}\\
= – \dfrac{{5 – 2\sqrt {35} + 7}}{2}\\
= – \dfrac{{12 – 2\sqrt {35} }}{2}\\
= – 6 + \sqrt {35}
\end{array}\)