An equation is a statement of an equality between two mathematical expressions to denote a particular relation between two quantities. Equations are used in most areas of physics, biochemistry and other allied sciences. Any equation involves one or more unknown quantities which can either be variables or indeterminates. These are denoted by letters, usually from among the last in the English alphabet. Greek letters can also be used. Apart from variables, an equation also has constant values. The main forms of equations are polynomial, algebraic, trigonometric, logarithmic and exponential.