Mathematics
Grade 11
15 min
Find the axis of symmetry of a parabola
Find the axis of symmetry of a parabola
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1
Introduction & Learning Objectives
Learning Objectives
Define the axis of symmetry and its relationship to the vertex of a parabola.
Derive the equation for the axis of symmetry from a quadratic function in standard form, y = ax^2 + bx + c.
Identify the equation for the axis of symmetry directly from a quadratic function in vertex form, y = a(x-h)^2 + k.
Calculate the equation for the axis of symmetry from a quadratic function in factored form, y = a(x-p)(x-q).
Correctly write the axis of symmetry as a vertical line equation (x = constant).
Visually identify and graph the axis of symmetry for a given parabola.
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This tutorial will teach you how to find the axis of symmetry, the invisible...
2
Key Concepts & Vocabulary
TermDefinitionExample
ParabolaThe characteristic U-shaped curve produced by a quadratic function (an equation of the form y = ax^2 + bx + c).The graph of y = x^2 is a simple parabola that opens upwards with its turning point at the origin (0,0).
Axis of SymmetryA vertical line that passes through the vertex of a parabola, dividing it into two congruent, mirror-image halves. Its equation is always in the form x = c, where c is a constant.For the parabola y = x^2, the axis of symmetry is the y-axis, which has the equation x = 0.
VertexThe highest or lowest point on a parabola, also known as the turning point. The vertex always lies on the axis of symmetry.In y = (x-3)^2 + 5, the vertex is at the point (3, 5).
Standard FormA quadratic function written as y = ax^2 + bx + c, where a, b, and c...
3
Core Formulas
Axis of Symmetry from Standard Form
For y = ax^2 + bx + c, the axis of symmetry is the line x = -b / (2a)
Use this formula when the quadratic equation is expanded. Identify the coefficients 'a' and 'b' and substitute them into the formula to find the x-value where the line of symmetry is located.
Axis of Symmetry from Vertex Form
For y = a(x-h)^2 + k, the axis of symmetry is the line x = h
Use this when the equation is in vertex form. The axis of symmetry is simply the x-coordinate of the vertex, 'h'. Be mindful of the sign inside the parentheses.
Axis of Symmetry from Factored Form
For y = a(x-p)(x-q), the axis of symmetry is the line x = (p + q) / 2
Use this when the equation is factored. The axis of symmetry is located exactly halfway...
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Challenging
A parabola has an axis of symmetry at x = 4 and one x-intercept at x = 1. What is the other x-intercept?
A.x = 5
B.x = 7
C.x = 3
D.x = -2
Challenging
The equation y = 2x^2 + bx - 7 describes a parabola with an axis of symmetry at x = -1. What is the value of b?
A.b = -4
B.b = -2
C.b = 2
D.b = 4
Challenging
A parabola has x-intercepts at (k - 5, 0) and (k + 9, 0) for some constant k. What is its axis of symmetry?
A.x = k
B.x = 7
C.x = k - 2
D.x = k + 2
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