b) (căn bậc hai 28 - 2 căn bậc hai 3 + căn bậc hai 7) căn bậc hai 7 + căn bậc hai 84

* Hướng dẫn giải

b) $\left(\sqrt{28}-2\sqrt{3}+\sqrt{7}\right)\sqrt{7}+\sqrt{84}$

$=\left(\sqrt{2^2.7}-2\sqrt{3}+\sqrt{7}\right)\sqrt{7}+\sqrt{2^2.21}$

$$\begin{gathered} =2\sqrt{7}.\sqrt{7}-2\sqrt{3}.\sqrt{7}+\sqrt{7}.\sqrt{7}+2\sqrt{21} \\ =2.7-2\sqrt{21}+7+2\sqrt{21} \\ =14+7 \\ =21 \end{gathered}$$