Cho các số \(3\sqrt 8 ;\;\,2\sqrt a ;\;\,7\sqrt 3 \). Khi đó:

 

Đáp án: 8

\(B = \frac{3}{{\sqrt 5 - \sqrt 2 }} + \frac{4}{{\sqrt 6 + \sqrt 2 }} + \frac{1}{{\sqrt 6 + \sqrt 5 }}\)

\( = \frac{{3\left( {\sqrt 5 + \sqrt 2 } \right)}}{{\left( {\sqrt 5 - \sqrt 2 } \right)\left( {\sqrt 5 + \sqrt 2 } \right)}} + \frac{{4\left( {\sqrt 6 - \sqrt 2 } \right)}}{{\left( {\sqrt 6 + \sqrt 2 } \right)\left( {\sqrt 6 - \sqrt 2 } \right)}} + \frac{{\sqrt 6 - \sqrt 5 }}{{\left( {\sqrt 6 + \sqrt 5 } \right)\left( {\sqrt 6 - \sqrt 5 } \right)}}\)

\( = \frac{{3\left( {\sqrt 5 + \sqrt 2 } \right)}}{{{{\left( {\sqrt 5 } \right)}^2} - {{\left( {\sqrt 2 } \right)}^2}}} + \frac{{4\left( {\sqrt 6 - \sqrt 2 } \right)}}{{{{\left( {\sqrt 6 } \right)}^2} - {{\left( {\sqrt 2 } \right)}^2}}} + \frac{{\sqrt 6 - \sqrt 5 }}{{{{\left( {\sqrt 6 } \right)}^2} - {{\left( {\sqrt 5 } \right)}^2}}}\)

\( = \frac{{3\left( {\sqrt 5 + \sqrt 2 } \right)}}{3} + \frac{{4\left( {\sqrt 6 - \sqrt 2 } \right)}}{4} + \frac{{\sqrt 6 - \sqrt 5 }}{1}\)

\( = \sqrt 5 + \sqrt 2 + \sqrt 6 - \sqrt 2 + \sqrt 6 - \sqrt 5 \)

\( = 2\sqrt 6 \).

Do đó, \(a = 2,\,\,b = 6\).

Vậy \(T = a + b = 8\).

Đáp án đúng là: A

Với \( - 1 < a < 1\), ta có

\(\left( {\frac{3}{{\sqrt {1 + a} }} + \sqrt {1 - a} } \right):\left( {\frac{3}{{\sqrt {1 - {a^2}} }} + 1} \right)\)

\( = \frac{{3 + \left( {\sqrt {1 + a} } \right)\left( {\sqrt {1 - a} } \right)}}{{\sqrt {1 + a} }}:\frac{{3 + \sqrt {1 - {a^2}} }}{{\sqrt {1 - {a^2}} }}\)

\( = \frac{{3 + \sqrt {1 - {a^2}} }}{{\sqrt {1 + a} }} \cdot \frac{{\sqrt {1 - {a^2}} }}{{3 + \sqrt {1 - {a^2}} }}\)

\( = \frac{{\sqrt {1 - {a^2}} }}{{\sqrt {1 + a} }}\)\( = \frac{{\left( {\sqrt {1 + a} } \right)\left( {\sqrt {1 - a} } \right)}}{{\sqrt {1 + a} }}\)\( = \sqrt {1 - a} \).