分之=√2(√5+√7)-√3(√5+√7)
=(√5+√7)(√2-√3)
同理,分母=(√5+√7)(√2+√3)
所以原式=(√2-√3)/(√2+√3)
=(√2-√3)²/(√2-√3)(√2+√3)
=(2-2√6+3)/(2-3)
=2√6-5