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Equations and identities

An equation is a statement with an equals sign, stating that two expressions are equal in value, for example: \(3x + 5 = 11\).

Solving an equation means finding the value or values for which the two expressions are equal. This means equations are not always true. In the example above, \(3x + 5 = 11\), the only correct solution for \(x\) is 2.

An identity is an equation which is always true, no matter what values are substituted. \(2x + 3x = 5x\) is an identity because \(2x + 3x\) will always simplify to \(5x\) regardless of the value of \(x\). Identities can be written with the sign ≡, so the example could be written as \(2x + 3x ≡ 5x\).

Example

Show that \(x = 2\) is the solution of the equation \(3x + 5 = 11\)

BIDMAS means the multiplication is carried out before the addition:

\(3x + 5 = 3 \times 2 + 5 = 6 + 5 = 11\)

Question

Say whether each of the following is an identity or an equation

  • \(5x + 10 = 3x + 8\)
  • \(5x + 10 = 5(x + 2)\)
  • \(5x + 10 = 5x +2\)