Equation 2 has a lot to say about the relationship between object distance and image distance, too. For a particular mirror, the focal length, f, is fixed. This means that if Do (object distance) is large, then Di (image distance) is small, and vice versa. Also, Do and Di must always have the same sign - either both positive or both negative. Physically, this means that the object is placed farther from the mirror than the focus, then the image is located beyond the focus also (both positive), and if the object is between the focus and the mirror, then the image is located on this side of the focus, too (both negative). This is precisely what we found from the ray diagrams.
Also, notice from the graph of equation 2 (at right) that if the object is located far from the focus, (Do > 20 cm, say), a relatively large movement of the object will produce only a very small movement of the image. When the object moves from Do = 40 cm to Do = 20 cm, the image only moves a slight distance away from the focus!