4a-3b-1=0
a=(3b+1)/4
根号(a2+b2)=(1/4)*根号[(3b+1)^2+16b^2]
=(1/4)*根号[(5b+(3/5))^2+(16/25)]>=(1/4)*根号(16/25)=1/5
所以,根号a2+b2的最小值=1/5