y=x*x-ax+2b*b过(1,3)即3=1-a+2b^22b^2-a-2=02b^2=a+2a*a+2b*b=a^2+2b^2=a^2+a+2=(a+1/2)^2-1/4+2=(a+1/2)^2+7/4大于等于7/4所以a*a+2b*b最小值为7/4
