If (mi,1mi) , i = 1,2,3,4 are concyclic points then the value of m1m2m3m4  is

x2+y2=r2  since (mi,1mi) lie on the circle ⇒m2+1m2=r2⇒m4−r2m2+1=0 whose roots are m1,m2,m3,m4 then product of rootsm1m2m3m4=S4=1