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Difference of two squares (DOTS)
\[ a^2 - b^2 = (a + b)(a - b) \]
The expressions \(a + b\) and \(a - b\) are called conjugates.

Conjugates are formed by changing the sign between two terms.

More examples:

ExpressionConjugate
\(x+y\)\(x-y\)
\(x+3\)\(x-3\)
\(x+\sqrt{5}\)\(x-\sqrt{5}\)

Factorise the following expressions:

Exercise #1

\[ \,\,\, 1) \quad x^2 - 100 \] \[ \,\,\, 2) \quad x^2 - 121 \] \[ \,\,\, 3) \quad x^2 - 169 \] \[ \,\,\, 4) \quad x^2 - 4 \] \[ \,\,\, 5) \quad x^2 - 64 \]

\[ \,\,\, 1) \quad x^2 - 100 = (x + 10)(x - 10) \] \[ \,\,\, 2) \quad x^2 - 121 = (x + 11)(x - 11) \] \[ \,\,\, 3) \quad x^2 - 169 = (x + 13)(x - 13) \] \[ \,\,\, 4) \quad x^2 - 4 = (x + 2)(x - 2) \] \[ \,\,\, 5) \quad x^2 - 64 = (x + 8)(x - 8) \]

Exercise #2

\[ \,\,\, 1) \quad x^2 - 95 \] \[ \,\,\, 2) \quad x^2 - 58 \] \[ \,\,\, 3) \quad x^2 - 7 \] \[ \,\,\, 4) \quad x^2 - 20 \] \[ \,\,\, 5) \quad x^2 - 88 \]

\[ \,\,\, 1) \quad x^2 - 95 = (x + \sqrt{95})(x - \sqrt{95}) \] \[ \,\,\, 2) \quad x^2 - 58 = (x + \sqrt{58})(x - \sqrt{58}) \] \[ \,\,\, 3) \quad x^2 - 7 = (x + \sqrt{7})(x - \sqrt{7}) \] \[ \,\,\, 4) \quad x^2 - 20 = (x + \sqrt{20})(x - \sqrt{20}) \] \[ \,\,\, 5) \quad x^2 - 88 = (x + \sqrt{88})(x - \sqrt{88}) \]

Quadratics