We have proton and electron and distance between them is $d$. If we choose $z$ axis along this dipole, net electric potential created by dipole on the point A is equal to summation of the electric potential created by electron and proton. If we defined the distance between point A and electron as $r_1$ and distance between point A and proton is defined by $r_2$. Then, we choose a middle point of the dipole and distance between this point and A is $r$, and angle between $z$ axis and $r$ is called $\alpha$. When we are in this step, we can approximate for $r_2$ minus $r_1$. I could not understand this approximation. How we can defined as below? How it is become?
$$ r_2-r_1 \approx d\cos \alpha$$
If my question is not clear please inform me.