a^2+b^2+5/4=2a+b
a^2-2a+1+b^2-b+1/4=0
(a-1)^2+(b-1/2)^2=0
(a-1)^2>=0；(b-1/2)^2>=0,
故：a=1,b=1/2
a+b=1+1/2=3/2
a*b=1*1/2=1/2
ab/(a+b)=(1/2)/(3/2)=1/3