slight tweaks to formatting of circle equations

In equations of the form x^2-2ax+y^2-2bx=C, if C is negative, then it gets put on the other side as x^2-2ax+y^2-2bx-C=0
added helper function to simplify surds of the form (a+b sqrt c)/d

This is handy for the circle/square intersection questions, which often have messy answers which we need to simplify.
finished mathmo question for circle/line intersections