@ Consider y defined as a function of x by the equation: Ax+By+C=0 If A, B and C are constants, show that the graph of y is a straight line. What happens when B=0? Note that we say that the locus of the point (x, y) is a straight line. The equation is the general equation of a line on a plane, and is a linear equation. An equation of the form (x-)+(y-B)= p is non-linear and represents a circle. Where is its centre? What is its radius? When does x+ y+2Gx+2Fy+C=0 represent a circle? This is a non-linear relationship between y and x. Can you express y in terms of x?