Firstly, determine the potential at the vertex which is sandwiched between the two equal edges (which subtend an angle $2 \alpha$ ) of an isosceles triangle with uniform surface charge density $\sigma$, if the height of the triangle from this vertex is $h$. Using this result, determine the potential at the center and thus the potential of a corner of a square with edge length $l$ and a uniform surface charge density $\sigma$.