 When the magnetic field is due to a straight wire, let's say this is the straight wire, it has a current of IE. I want to find out the magnetic field at a distance z coming, z is a point that two ends, main and middle, let's say theta point is making, it's a two end, okay? Are you similar in diagram, you have seen somewhere, when we found out something, smaller in the element somewhere, drop, all vector and draw it again and it's two lines, it will be cleaner, there is no concentration everywhere it is. So D means theta, that's a D end, so I have to write down everything in terms of theta only, okay? So can you write R equals z? Zc theta, R equals small square, this is z, only by differential, sine theta, you have to put limits minus theta 1 and plus theta 2, so what we get? So this is the theta at the set of the triangle, this is the equilibrium time set, three such line segments are there, understood?