text: Use lens maker's formula\n1/f = (n-1)(1/R1-1/R2)\nBut use n=-1 as the "refractive index" as light travels with same speed but in opposite direction\nSo f=0.5*R1*R2/(R2-R1)        (here R1 is  -ve and R2 is +ve) \n    f= 0.857 m\nsince the star is at "infinity" , the light rays will focus at 0.857m from convex mirror or 0.107 m from concave