let AB be a straight line parallel to x axis. then the ordinate of every point on lineAB is constant. let it be assumed b.
let p(x, y) be any point on line AB. the line y=b is required equation of line AB. the line y=b lies above or below x-axis according as b is positive or negative.
if b=0 then line AB coincides with x axis. so the equation of x-axis is <u>y</u><u>=</u><u>0</u>.
