Question

Tính

A .\(2\sqrt{20}\)-\(\sqrt{45}\)+\(\dfrac{2}{5}\sqrt{125}\)                           B.\(\dfrac{4}{3-\sqrt{5}}-\dfrac{4}{3+\sqrt{5}}\)

Answer 1

\(A.2\sqrt{20}-\sqrt{45}+\dfrac{2}{5}\sqrt{125}\)

\(=4\sqrt{5}-3\sqrt{5}+2\sqrt{5}\)

\(=3\sqrt{5}\)

\(B.\dfrac{4}{3-\sqrt{5}}-\dfrac{4}{3+\sqrt{5}}\)

\(=3+\sqrt{5}+3-\sqrt{5}\)

\(=3+3=6\)

Answer 2

`a)2\sqrt{20}-\sqrt{45}+2/5\sqrt{125}`

`=2\sqrt{2^2 .5}-\sqrt{3^2 .5}+2/5\sqrt{5^2 .5}`

`=4\sqrt{5}-3\sqrt{5}+2\sqrt{5}=3\sqrt{5}`

`b)4/[3-\sqrt{5}]+4/[3+\sqrt{5}]`

`=[4(3+\sqrt{5})+4(3-\sqrt{5})]/[9-5]`

`=[4(3+\sqrt{5}+3-\sqrt{5})]/4=6`