原式=[√2(√5+√7)-√3(√5+√7)]/[√2(√5+√7)+√3(√5+√7)]=[(√2-√3)(√5+√7)]/[(√2+√3)(√5+√7)]=(√2-√3)/(√2+√3)=(√2-√3)²/[(√2-√3)(√2+√3)]=(2-2√6+3)/(2-3)=2√6-5
