The linear magnification of a concave mirror is `m =(h_(2))/(h_(1))= "size of image"/"size of object" = v/u`

Clearly, `m gt 1`, when image is enlarged, and `mlt1`, when image is smaller in size than the object. Further, when image is inverted and real, `h_(2)` is negative, `h_(1)` is positive. Therefore, m is negative, when image is erect and virtual, `h_(2)` is positive, `h_(1)` is positive. Therefore, m is positive..