Rút gọn biểu thức
a. \(\left(2\sqrt{5}-\sqrt{7}\right)\left(2\sqrt{5}+\sqrt{7}\right)\)
b.\(\left(5\sqrt{2}+2\sqrt{3}\right)\left(2\sqrt{3}-5\sqrt{2}\right)\)
c. \(\sqrt{9+4\sqrt{5}}\)
d. \(\sqrt{14-6\sqrt{5}}+\sqrt{14+6\sqrt{5}}\)
e. \(\sqrt{55-6\sqrt{6}}\)
f. \(\sqrt{21-6\sqrt{6}}\)