Optics did my professor screw up?

[Dart82]

Homework Statement

A cat is standing 1m in front of a mirror. a virtual image is formed 10.0m behind the mirror. what is the radius of curvature for the mirror?

Homework Equations

1/f = (1/do)+(1/di)
f=+/-(1/2)*R

The Attempt at a Solution

(1/1m + 1/10m)^-1 = .909
.909*2 = 1.82

According to my answer key, R should be equal to 2.2m. the only way i get that answer is if i take (1/do)+(1/di) and not take its reciprocal...i would just plug it straight into the focal length equation to solve for R. Did he make a mistake??

 

[nrqed]

Homework Statement

A cat is standing 1m in front of a mirror. a virtual image is formed 10.0m behind the mirror. what is the radius of curvature for the mirror?

Homework Equations

1/f = (1/do)+(1/di)
f=+/-(1/2)*R

The Attempt at a Solution

(1/1m + 1/10m)^-1 = .909
.909*2 = 1.82

According to my answer key, R should be equal to 2.2m. the only way i get that answer is if i take (1/do)+(1/di) and not take its reciprocal...i would just plug it straight into the focal length equation to solve for R. Did he make a mistake??

The image is virtual so di should be taken to be negative.

 

[DAKONG]

you miss substitution
di = -10.0 m