Chapter 10: Problem 73
Find the vertex, axis of symmetry, \(x\) -intercept, \(y\) -intercepts, focus, and directrix for each parabola. Sketch the graph, showing the focus and directrix. $$x=2(y-1)^{2}+3$$
Short Answer
Expert verified
Vertex: (3, 1). Axis of symmetry: y = 1. x-intercept: (5, 0). No y-intercept. Focus: (3.125, 1). Directrix: x = 2.875.
Step by step solution
01
- Identify the vertex form
Start by rewriting the given equation in a more familiar form. Notice that the equation is already in the form of a parabola: \[ x = a(y - k)^2 + h \] where \(a=2\), \(h=3\), and \(k=1\).
02
- Determine the vertex
In the parabola equation \[ x = 2(y - 1)^2 + 3 \], the vertex can be identified as the point \( (h, k) = (3, 1) \).
03
- Find the axis of symmetry
The axis of symmetry for the parabola opens to the right (or left) and is vertical. It passes through the vertex's y-coordinate: \[ y = 1 \].
04
- Calculate the x-intercept
Set \(y = 0\) and solve for \( x \): \[ x = 2(0 - 1)^2 + 3 \] \[ x = 2(1) + 3 \] \[ x = 5 \] Therefore, the x-intercept is \( (5, 0) \).
05
- Calculate the y-intercept
Set \(x = 0\) and solve for \( y \): \[ 0 = 2(y - 1)^2 + 3 \] \[ -3 = 2(y - 1)^2 \] \[ -\frac{3}{2} = (y - 1)^2 \] This equation has no real solutions since the left-hand side is negative and a square is always non-negative. Hence, there is no y-intercept.
06
- Identify the focus
For the parabola \[ x = 2 (y - 1)^2 + 3 \], find the focus using the formula for horizontal parabolas: \[ (h + \frac{1}{4a}, k) \] where \(a = 2\). Using this: \[ h + \frac{1}{8} = 3 + \frac{1}{8} \] So, the focus is \( (3.125, 1) \).
07
- Determine the directrix
The directrix for a horizontal parabola given by \( x = 2 (y - 1)^2 + 3 \) can be determined using: \[ x = h - \frac{1}{4a} \] where \(a = 2\). Using this: \[ x = 3 - \frac{1}{8} = 2.875 \] Thus, the directrix is \( x = 2.875 \).
08
- Sketch the graph
Using the vertex, focus, and directrix, sketch the parabola. The vertex is at (3, 1), the axis of symmetry is \( y = 1 \), the focus is at (3.125, 1), and the directrix is the vertical line \( x = 2.875 \). The parabola opens to the right.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Axis of Symmetry
The axis of symmetry for a parabola is a vertical line that divides the parabola into two mirror-image halves. For parabolas of the form \[ x = a(y - k)^2 + h \], the axis of symmetry is given by the equation \[ y = k \]. This line passes through the vertex and ensures that both sides of the parabola are symmetrical. In our specific example, the equation given is \[ x = 2(y - 1)^2 + 3 \]. Here, the vertex \[ (h, k) \] is \[ (3, 1) \], meaning the axis of symmetry is \[ y = 1 \]. This means the parabola is symmetrical about the line where the y-coordinate is always 1.
Focus and Directrix
The focus and directrix are two important properties of a parabola. The focus lies inside the parabola, while the directrix is a line outside of it. Points on the parabola are such that the distance to the focus equals the distance to the directrix. For a horizontal parabola such as \[ x = 2 (y - 1)^2 + 3 \], the coordinates of the focus are given by: \[ \left( h + \frac{1}{4a}, k \right) \], where \[ a = 2 \]. Substituting the given values: \[ h + \frac{1}{8} = 3 + \frac{1}{8} = 3.125 \]. Hence, the focus is \[ (3.125, 1) \].
The directrix for the horizontal parabola can be found using: \[ x = h - \frac{1}{4a} \]. Substituting the values, we get: \[ x = 3 - \frac{1}{8} = 2.875 \]. Therefore, the directrix is \[ x = 2.875 \]. These two features help in sketching the parabola accurately.
The directrix for the horizontal parabola can be found using: \[ x = h - \frac{1}{4a} \]. Substituting the values, we get: \[ x = 3 - \frac{1}{8} = 2.875 \]. Therefore, the directrix is \[ x = 2.875 \]. These two features help in sketching the parabola accurately.
x- and y-intercepts
The intercepts are where the parabola crosses the x-axis and y-axis. For the x-intercept, set \[ y = 0 \] and solve for \[ x \]: \[ x = 2(0 - 1)^2 + 3 \], simplifies to: \[ x = 2(1) + 3 = 5 \]. Therefore, the x-intercept is \[ (5, 0) \].
To find the y-intercept, set \[ x = 0 \] and solve for \[ y \]: \[ 0 = 2(y - 1)^2 + 3 \]. This rearranges into: \[ -3 = 2(y - 1)^2 \] \[ -\frac{3}{2} = (y - 1)^2 \]. Since a square is always non-negative and the left side is negative, the equation has no real solutions. This means the parabola does not intersect the y-axis and has no y-intercept.
To find the y-intercept, set \[ x = 0 \] and solve for \[ y \]: \[ 0 = 2(y - 1)^2 + 3 \]. This rearranges into: \[ -3 = 2(y - 1)^2 \] \[ -\frac{3}{2} = (y - 1)^2 \]. Since a square is always non-negative and the left side is negative, the equation has no real solutions. This means the parabola does not intersect the y-axis and has no y-intercept.