Find the vertex, focus, axis, directrix and latus-rectum of the following parabolas
y2 − 4y − 3x + 1 = 0
Solution
Given:
y2 − 4y − 3x + 1 = 0
\[\Rightarrow \left( y - 2 \right)^2 - 4 - 3x + 1 = 0\]
\[ \Rightarrow \left( y - 2 \right)^2 = 3\left( x + 1 \right)\]
\[ \Rightarrow \left( y - 2 \right)^2 = 3\left( x - \left( - 1 \right) \right)\]
Let \[Y = y - 2\]
\[X = x + 1\]
Then, we have:
\[Y^2 = 3X\]
Comparing the given equation with\[Y^2 = 4aX\]
\[4a = 3 \Rightarrow a = \frac{3}{4}\]
∴ Vertex = (X = 0, Y = 0) = \[\left( x = - 1, y = 2 \right)\]
Focus = (X = a, Y = 0) = \[\left( x + 1 = \frac{3}{4}, y - 2 = 0 \right) = \left( x = \frac{- 1}{4}, y = 2 \right)\]
Equation of the directrix:
X = −a
i.e. \[x + 1 = \frac{- 3}{4} \Rightarrow x = \frac{- 7}{4}\]
Axis = Y = 0
i.e. \[y - 2 = 0 \Rightarrow y = 2\]
Length of the latus rectum = 4a = 3 units