Radius of Curvature of Convex Mirror: -17.39 cm

[Niendel]

A real object is placed at the zero end of a meterstick. A large concave mirror at the 100 cm end of the meterstuck forms an image of the object at the 82.4 cm position. A small convex mirror placed at the 20 cm pisition form a final image at the 6.3 cm point. What is the radius of curvature of the convex mirror? Answer in unites of cm.

So i know that since i have 2 mirrors the image for the 1st image will be the object for the 2nd mirror.

Therefore Di= -13.6 cm
Do=62.4 cm

since 1/do + 1/di = 1/f
f = -17.39
therefore Since r=2F
r should be -17.39 cm according to my calculations but I am doing something wrong cause this is not the answer!

 

[member 553083]

Can someone please explain to me what I am doing wrong here? You are forgetting that the focal length of a convex mirror is positive. So, instead of 1/do + 1/di = 1/f, you should use 1/do - 1/di = 1/f. This means that f = 34.78 cm and r = 69.56 cm.