ta có

A=\(\left(\sqrt{10}-\sqrt{2}\right)\sqrt{3+\sqrt{5}}\)

=\(\sqrt{10}.\sqrt{3+\sqrt{5}}-\sqrt{2}.\sqrt{3+\sqrt{5}}\)

=\(\sqrt{10.\left(3+\sqrt{5}\right)}-\sqrt{2.\left(3+\sqrt{5}\right)}=\sqrt{30+10\sqrt{5}}-\sqrt{6+2\sqrt{5}}\)

=\(\sqrt{5^2+2.5.\sqrt{5}+\left(\sqrt{5}\right)^2}-\sqrt{\left(\sqrt{5}\right)^2+2.1.\sqrt{5}+1}\)

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

thấy bình thường quan trọng là việc trả lời đúng hay ko thôi