Equation of circle having centre of origin (0,0) and radius =r,
S1;x2+y2=r2−−−−(1)
∴f(m1,1m2),f(m2,1m2),−−f(m4,1m4)
These points lie on S1.
Let f(m,1m) is point lie on S_{1},
m2+1m2=r2
m4+1−r2m2=0
m4−1−r2m2+1=0
m1,m2,m3 and m4 are roots of this equation
So, m1m2m3m4=1