An infinite number of tangents can be drawn from $(1,2)$   to the circle $x^2+y^2-2x-4y+\lambda =0$  then the value of  $\lambda$  =

Clearly the point (1,2) is the centre and infinite tangents can be drawn only on a point circle   $\to r=0 \therefore \sqrt{1^2+2^2-\lambda }=0\Rightarrow \lambda =5$