a=2/(√7+√5)=2(√7-√5)/[(√7+√5)(√7-√5)]=√7-√5;b=2/(√7-√5)=2(√7-√5)/[(√7+√5)(√7-√5)]=√7+√5;所以：a+b=2√7ab=7-5=2.3a^2-ab+3b^2=3(a^2+b^2)-ab=3(a^2+2ab+b^2)-7ab=3(a+b)^2-7ab=70.
