由原试解出a=(b+1-b*2)/2b带入a+2b=(b+1-b*2)/2b+2b=(4b*-b*2+b+1)/2b=(3b*2+b+1)/2b=3/2b+1/2+1/2b=1/2(3b+1/b)+1/2>=1/2(2根号下3bx1/b)+1/2=根号3+1/2当且仅当3b=1/b时,即b=根号3/3时取等号
