Theory of Intelligence
11TH•Mathematics•Functions And Graphs•Question 4

(v) $g(x) = \dfrac{x + 2}{3 - x}$

Find the domain and the range of the function $g$ defined below:

(v) g(x)=x+23−xg(x) = \dfrac{x + 2}{3 - x}

Given:

g(x)=x+23−xg(x) = \frac{x + 2}{3 - x}
  • Domain (DgD_g): Denominator cannot equal zero (3−x≠0  ⟹  x≠33 - x \ne 0 \implies x \ne 3):

    • g(−2)=−2+23−(−2)=05=0g(-2) = \dfrac{-2 + 2}{3 - (-2)} = \dfrac{0}{5} = 0
    • g(0)=0+23−0=23g(0) = \dfrac{0 + 2}{3 - 0} = \dfrac{2}{3}
    • For x=3x = 3: 3+23−3=50\dfrac{3 + 2}{3 - 3} = \dfrac{5}{0} (Undefined)
    Dg=R∖{3}=(−∞,3)∪(3,∞)\boxed{D_g = \mathbb{R} \setminus \{3\} = (-\infty, 3) \cup (3, \infty)}
  • Range (RgR_g): Let y=x+23−xy = \dfrac{x + 2}{3 - x} and solve for xx:

    y(3−x)=x+23y−xy=x+23y−2=x(1+y)x=3y−21+y\begin{aligned} y(3 - x) &= x + 2 \\ 3y - xy &= x + 2 \\ 3y - 2 &= x(1 + y) \\ x &= \frac{3y - 2}{1 + y} \end{aligned}

    For xx to be defined, require 1+y≠0  ⟹  y≠−11 + y \ne 0 \implies y \ne -1:

    • For y=2  ⟹  x=3(2)−21+2=43  ⟹  g(4/3)=2y = 2 \implies x = \dfrac{3(2) - 2}{1 + 2} = \dfrac{4}{3} \implies g(4/3) = 2
    Rg=R∖{−1}=(−∞,−1)∪(−1,∞)\boxed{R_g = \mathbb{R} \setminus \{-1\} = (-\infty, -1) \cup (-1, \infty)}
Dg=R∖{3},Rg=R∖{−1}\boxed{D_g = \mathbb{R} \setminus \{3\}, \quad R_g = \mathbb{R} \setminus \{-1\}}

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