For family of circles whose inscribed angle makes an angle of $ \theta$ with a line segment is given as:
If the coordinate of $E=(x_1,y_1)$ and of $B=(x_2,y_2)$ then the family of circles for which $EBH$ makes an angle $\theta$ (here 53) is given as: $$(x-x_1)(x-x_2) + (y-y_1)(y-y_2) \pm \cot \theta \left[ (x-x_1)(y-y_2) - (x-x_2)(y-y_1)\right]$$
This result was given a JEE book (Famous Indian engineering exam) with no explanation why this is true, could any one shed light on why this formula represents what it is supposed to?
