答：
1/[(7-4√3)*(7+4√3)]
=1/(49-48)
=1/1
=1
3/(√2-√3)
=3(√2+√3)/[(√2+√3)(√2-√3)]
=3(√2+√3)/(2-3)
=-3√2-3√3
(a-b)/(√a+√b)
=(√a+√b)(√a-√b)/(√a+√b）
=√a-√b