Hello! For Single Charge: I studied the formula for electric potential for a point charge (V=kq/r). It was derived when the test charge approached the source charge from infinity "head-on". In this case the electric field due to source charge and displacement of test charge were vectors in same(or at 180degrees) direction. Fine. For Multiple Charge: Now, suppose there's a cube with +q point charges at its vertices. Now, to calculate the potential, let's say at the centre of the cube we add potentials due to all those eight charges using v=kq/r where we put r as the distance between vertice and centre of cube. But this should work only if the test charge brought from infinity is head-on to every charge at vertice of cube! And you cannot do that since in this arrangement, if you try to bring the test charge from infinity head on to any one charge out of eight, you will start having an angle between other six charges. Hence, we must take account of cos theta and stuff because then electric field (of source) and displacement (of test charge) are at an angle to each other. So why do we straightaway use that formula?