(a+b)(a-b)=12设x=a+b,y=a-b,则a=(x+y)/2,b=(x-y)/2,xy=12a^2+b^2+ab=(x+y)²/4+(x-y)²/4+(x+y)(x-y)/4=(1/4)(2x²+2y²+x²-y²)=(1/4)(3x²+y²)>=(1/4)2√(3x²y²)=(...