**Constant Function: **Let k be a fixed real number. Then a function f (x) given by f (x) = k for all x ϵ R is called a constant function. Sometimes we also call it the constant function k.**Identity Function: **The function defined by I (x) = x for all x ϵ R, is called the identity function on R.**Modulus Function: **The function defined by f (x) = |x| \(=\left\{ \begin{align}&x,\,\,\,when\,\,\,x\ge 0 \\& -x,\,\,when\,\,\,x<0 \\\end{align} \right.\) is called the modulus function.**Properties of Modulus Function: **The modulus function has the following properties:

i) For any real number x we have √x^{2} = |x|.

ii) If a, b are positive real numbers then

⇒ x^{2 }≤ a^{2} |x| ≤ a ⇔ -a ≤ x ≤ a

⇒ x^{2 }≥ a^{2} ⇔|x| ≥ a ⇔ x ≤ -a or x ≥ a

⇒ x^{2 }> a^{2} ⇔|x| > a ⇔ x < -a or x > a

⇒ a² ≤ x² ≤ b² ⇔ a ≤ |x| ≤ b ⇔ x ϵ [-b, -a] U [a, b]

⇒ a^{2} ≤ x^{2} ≤ b^{2 }⇔ a ≤ |x| ≤ b ⇔ x ϵ [-b, -a] U [a, b]

iii) |x + y| = |x| + |y|

⇒ (x ≥ 0 and y ≥ 0) or (x < 0 and y < 0)

iv) |x – y| = |x| – |y|

⇒ (x ≥ 0) and |x| ≥ |y| or (x ≤ 0, y ≤ 0 and |x| ≥ |y|)

v) |x ± y| < |x| + |y|

vi) |x ± y| ≥ ||x| – |y||