a) Đề sai rồi nha.
Làm đề sửa:[tex](\sqrt{5}+2)(\sqrt{17-4\sqrt{9+4\sqrt{5}}})=(\sqrt{5}+2)(\sqrt{17-4\sqrt{(\sqrt{5}+\sqrt{4})^2}})=(\sqrt{5}+2)(\sqrt{17-4(\sqrt{5}+2)})=(\sqrt{5}+2)(\sqrt{9-4\sqrt{5}})=(\sqrt{5}+2)(\sqrt{(\sqrt{5}-2)^2})=(\sqrt{5}+2)(\sqrt{5}-2)=5-4=1[/tex]
b)[tex]\sqrt{4-\sqrt{7}}.\sqrt{4+\sqrt{7}}=\sqrt{4^2-7}=3[/tex]