Math Problem Statement

Find the​ vertex, focus, and directrix of the following parabola. Graph the equation. x squared plus 2 x equals negative y plus 1

Solution

We are given the equation of the parabola:

x2+2x=y+1x^2 + 2x = -y + 1

Step 1: Rearrange the equation

To better analyze this equation, let's first rearrange it so that the yy-terms are on one side and the xx-terms are on the other:

x2+2x+y=1x^2 + 2x + y = 1

Now, move the constant to the other side:

x2+2x=y+1x^2 + 2x = -y + 1

y=x22x+1y = -x^2 - 2x + 1

Step 2: Complete the square

To rewrite the equation in a form that reveals the vertex, we need to complete the square for the xx-terms. The xx-terms in this equation are x2+2xx^2 + 2x.

  1. Take half of the coefficient of xx, which is 22, then square it: (22)2=1\left(\frac{2}{2}\right)^2 = 1.
  2. Add and subtract this number inside the equation:

y=(x2+2x+11)+1y = -(x^2 + 2x + 1 - 1) + 1

Now simplify:

y=(x+1)2+1y = -(x + 1)^2 + 1

So the equation becomes:

y=(x+1)2+1y = -(x + 1)^2 + 1

This is now in vertex form, where the vertex is at (1,1)(-1, 1).

Step 3: Identify the vertex, focus, and directrix

  1. Vertex: From the equation y=(x+1)2+1y = -(x + 1)^2 + 1, the vertex is (1,1)(-1, 1).

  2. Focus: The equation y=a(xh)2+ky = a(x - h)^2 + k has the general form of a parabola that opens up or down. For y=(x+1)2+1y = -(x + 1)^2 + 1, the parabola opens downward because the coefficient of (x+1)2(x + 1)^2 is negative.

    The standard form of a parabola is y=a(xh)2+ky = a(x - h)^2 + k, and the distance from the vertex to the focus is given by:

    Distance=14a\text{Distance} = \frac{1}{4a}

    Here, a=1a = -1, so:

    Distance=14(1)=14\text{Distance} = \frac{1}{4(-1)} = -\frac{1}{4}

    Since the parabola opens downward, the focus will be at a distance of 14\frac{1}{4} units below the vertex. The vertex is at (1,1)(-1, 1), so the focus is at:

    (1,114)=(1,34)\left(-1, 1 - \frac{1}{4}\right) = \left(-1, \frac{3}{4}\right)

  3. Directrix: The directrix is a horizontal line located at a distance of 14\frac{1}{4} units above the vertex. Since the vertex is at (1,1)(-1, 1), the directrix will be the line:

    y=1+14=54y = 1 + \frac{1}{4} = \frac{5}{4}

Step 4: Graph the parabola

The graph will have the following characteristics:

  • The vertex is at (1,1)(-1, 1).
  • The focus is at (1,34)(-1, \frac{3}{4}).
  • The directrix is the line y=54y = \frac{5}{4}.

I can help you graph it, or if you'd prefer, I can generate a visual for you. Let me know!


Would you like more details on how to plot the parabola or any other aspect of this problem?

Here are some related questions:

  1. How do I find the vertex of a parabola from its standard equation?
  2. How do you find the distance to the focus and the directrix?
  3. What is the difference between a parabola that opens upward versus downward?
  4. How do you complete the square when the coefficient of x2x^2 is negative?
  5. How can I apply this to other types of parabolas (sideways or vertically shifted)?

Tip: Completing the square is a key step when dealing with quadratic equations and helps convert a general equation into vertex form, making it easier to identify the vertex, focus, and directrix.

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Math Problem Analysis

Mathematical Concepts

Algebra
Parabolas
Quadratic Equations
Vertex Form

Formulas

y = a(x - h)^2 + k
Distance to focus = 1/(4a)

Theorems

Parabola Focus and Directrix Theorem

Suitable Grade Level

Grades 9-11