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  An equation is written as two expressions, connected by an equals sign ("="). The expressions on the two sides of the equals sign are called the "left-hand side" and "right-hand side" of the equation. Very often the right-hand side of an equation is assumed to be zero. This does not reduce the generality, as this can be realized by subtracting the right-hand side from both sides.
  
  The most common type of equation is a polynomial equation (commonly called also an algebraic equation) in which the two sides are polynomials.The sides of a polynomial equation contain one or more terms. For example, the equation
  
  has left-hand side A x 2 + B x + C − y {\displaystyle Ax^{2}+Bx+C-y}, which has four terms, and right-hand side 0 {\displaystyle 0}, consisting of just one term. The names of the variables suggest that x and y are unknowns, and that A, B, and C are parameters, but this is normally fixed by the context (in some contexts, y may be a parameter, or A, B, and C may be ordinary variables).
  
  An equation is analogous to a scale into which weights are placed. When equal weights of something (e.g., grain) are placed into the two pans, the two weights cause the scale to be in balance and are said to be equal. If a quantity of grain is removed from one pan of the balance, an equal amount of grain must be removed from the other pan to keep the scale in balance. More generally, an equation remains in balance if the same operation is performed on its both sides.