Let `f` be a mapping with domain `D` such that `y = f(x)` should assume angle value for each `x`, (i.e. the straight line drawn parallel to `y`-axis in its domain should cut at only one point.

Eg. `y = x^3`

Here all the straight lines parallel to `y`-axis cut `y = x^3` only at one print.

Eg. `x^2 + y^2 = 12`

Here line parallel to `y`-axis is intersecting the circle at two points hence it is not a function.

Domain :
Rule for finding Domain :

(i) Expression under even root (i.e. square root, fourth root etc) `>= 0`.

(ii) Denominator `!= 0`

(ill) If domain of `y =f(x) ` & `y =g(x)` are `D_1` & `D_2` respectively then the domain of `f(x) � g(x)` or `f(x). g(x)` is `D_1 nn D_2`

(iv) Domain of `(f(x))/(g(x))` is `D_1 nn D_2 - {g(x) =0}`