Question

If the points $(2,0),(0,1),(4,0)$ and $(0,a)$ are concyclic then $a=$

Answer 1

Let $A(2,0),B(0,1),C(4,0),D(0,a)$

$OA\cdot OC=OB\cdot OD$
$\Rightarrow 2\cdot 4=1\cdot a\Rightarrow a=8$