Circle a triangle whose base has the length of the circle's circumference and whose height equals the circle's radius, which comes to multiplied by the radius squared: Equivalently, denoting diameter by "d", that is, approximately 79% of the circumscribing square (whose side is of length "d"). The circle is the plane curve enclosing the maximum area for a given arc length. This relates the circle to a problem in the calculus of variations, namely the isoperimetric inequality. Equation of a circle<br> In an "x"–"y" Cartesian coordinate system, the circle with centre coordinates ("a", "b") and radius "r" is the set of
Distance from a point to a line equation of a line can be given in vector form: Here is the position of a point on the line, and is a unit vector in the direction of the line. Then as scalar "t" varies, gives the locus of the line. The distance of an arbitrary point to this line is given by This formula can be derived as follows: formula_39 is a vector from to the point on the line. Then formula_40 is the projected length onto the line and so is a vector that is the projection of formula_39 onto the line. Thus is the component of