A) căn(2+căn3)-căn(2-căn3)
B) căn(12-3căn7)-căn(12+3căn7)
a) $A=\sqrt{2+\sqrt{3}}-\sqrt{2-\sqrt{3}}$
$\Rightarrow A\sqrt{2}=\sqrt{4+2\sqrt{3}}-\sqrt{4-2\sqrt{3}}$
$=\sqrt{(\sqrt{3}+1)^2}-\sqrt{(\sqrt{3}-1)^2}$
$=\sqrt{3}+1-\sqrt{3}+1=2$
$\Rightarrow A=\sqrt{2}$
b) $B=\sqrt{12-3\sqrt{7}}-\sqrt{12+3\sqrt{7}}$
$\Rightarrow B\sqrt{2}=\sqrt{24-6\sqrt{7}}-\sqrt{24+6\sqrt{7}}$
$=\sqrt{(\sqrt{21}-\sqrt{3})^2}-\sqrt{(\sqrt{21}+\sqrt{3})^2}$
$=\sqrt{21}-\sqrt 3-\sqrt{21}-\sqrt 3$
$=-2\sqrt 3$
$\Rightarrow B=-\sqrt 6$