Ta có

A   =   5 ( x   +   4 ) 2   +   4 ( x   –   5 ) 2   –   9 ( 4   +   x ) ( x   –   4 )     =   5 ( x 2   +   2 . x . 4   +   16 )   +   4 ( x 2   –   2 . x . 5   +   5 2 )   –   9 ( x 2   –   4 2 )     =   5 ( x 2   +   8 x   +   16 )   +   4 ( x 2   –   10 x   +   25 )   –   9 ( x 2   –   4 2 )     =   5 x 2   +   40 x   +   80   +   4 x 2   –   40 x   +   100   –   9 x 2   +   144     = ( 5 x 2   +   4 x 2   –   9 x 2 )   +   ( 40 x   –   40 x )   +   ( 80   + 100   +   144 )

= 324

Đáp án cần chọn là: C

\(\dfrac{\sqrt{9-4\sqrt{5}}}{2-\sqrt{5}}=\dfrac{\sqrt{\left(\sqrt{5}-2\right)^2}}{2-\sqrt{5}}=\dfrac{\sqrt{5}-2}{2-\sqrt{5}}=\dfrac{-\left(2-\sqrt{5}\right)}{2-\sqrt{5}}=-1\)

\(\dfrac{\sqrt{9-4\sqrt{5}}}{2-\sqrt{5}}=-1\)

1) \(\sqrt{9+4\sqrt{5}}-\sqrt{9-4\sqrt{5}}\)

\(=\sqrt{2^2+2\cdot2\cdot\sqrt{5}+\left(\sqrt{5}\right)^2}-\sqrt{2^2-2\cdot2\cdot\sqrt{5}+\left(\sqrt{5}\right)^2}\)

\(=\sqrt{\left(2+\sqrt{5}\right)^2}-\sqrt{\left(2-\sqrt{5}\right)^2}\)

\(=\left|2+\sqrt{5}\right|-\left|2-\sqrt{5}\right|\)

\(=2+\sqrt{5}+2-\sqrt{5}\)

\(=4\)

2) \(\sqrt{12-6\sqrt{3}}+\sqrt{12+6\sqrt{3}}\)

\(=\sqrt{3^2-2\cdot3\cdot\sqrt{3}+\left(\sqrt{3}\right)^2}+\sqrt{3^2+2\cdot3\cdot\sqrt{3}+\left(\sqrt{3}\right)^2}\)

\(=\sqrt{\left(3-\sqrt{3}\right)^2}+\sqrt{\left(3+\sqrt{3}\right)^2}\)

\(=\left|3-\sqrt{3}\right|+\left|3+\sqrt{3}\right|\)

\(=3-\sqrt{3}+3+\sqrt{3}\)

\(=6\)

xin chào đáp án là 2 bạn ạ

1, 3^4

2, 5^405

3, -20/3

4, 4

                                                                             KHÔNG BÍT ĐÚNG HAY KO =>HIHIHIHI

10+1/4 

=41/4

b) 4 x 9 / 27 x 2 

= 2/3

10+\(\frac{1}{4}\)

=\(\frac{41}{4}\)

b) 4 x \(\frac{9}{27}\) x 2 

= \(\frac{2}{3}\)

\(\sqrt{\dfrac{\left(4-\sqrt{5}\right)^2a^2}{9}}\)

\(=\dfrac{\sqrt{\left(4-\sqrt{5}\right)^2a^2}}{\sqrt{9}}\)

\(=\dfrac{\left|\left(4-\sqrt{5}\right)a\right|}{3}\)

\(=\dfrac{\left(4-\sqrt{5}\right)\left|a\right|}{3}\)