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Quadratic graphs and their properties

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Apuntes de la lección

The shape of a quadratic graph

  • A quadratic has the form y = ax2 + bx + c with a \neq 0. Its graph is a smooth, symmetrical curve called a parabola.
  • If a > 0 the parabola is U-shaped with a minimum; if a < 0 it is ∩-shaped with a maximum.
  • The y-intercept is c, where x = 0.

Positive and negative leading coefficients

Positive and negative leading coefficientsO−6−4−2246810−3−2−1123xyy = 3 − x²y = x²

Roots and the line of symmetry

  • Real roots are the x-values where the graph meets the x-axis, so y = 0. It may cross the axis or touch it at a repeated root.
  • The parabola is symmetrical about a vertical line of symmetry halfway between the roots: x = \frac{x1 + x2}{2}, or x = -\frac{b}{2a}.
  • A quadratic may have two roots, one repeated root (the graph just touches the axis), or no real roots (the graph misses the axis).

Roots and turning point

Roots and turning pointO−2246810−112345xy(1, 0)(3, 0)y = x² −4x + 3Min (2, −1)

The turning point

  • The turning point (vertex) lies on the line of symmetry. Substitute that x-value into the equation to find y.
  • For y = x2 - 4x + 3: roots at x = 1 and x = 3, line of symmetry x = 2, turning point y = 4 - 8 + 3 = -1, so (2, -1).
  • Completed square form: y = (x - p)2 + q has turning point (p, q).

Drawing a quadratic

  • Make a table of values for several x values (include negatives), plot the points and join them with a smooth curve, not straight lines.
  • Check the curve is symmetrical and passes through the intercepts.

Diapositivas

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Preguntas de práctica

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  1. 1.What is the graph of a quadratic function called?

    Easy
    • AA hyperbola
    • BA parabola
    • CA circle
    • DA straight line
  2. 2.For y = x2 - 5, where does the graph cross the y-axis?

    Easy
    • A(0, 5)
    • B(0, -5)
    • C(5, 0)
    • D(-5, 0)
  3. 3.What shape is y = 2x2 + 1?

    Easy
    • A∩-shaped
    • BU-shaped
    • Ca straight line
    • Dan S-shape
  4. 4.What shape is y = 3 - x2?

    Easy
    • AU-shaped
    • B∩-shaped
    • Ca straight line
    • Da circle
  5. 5.What are the roots of y = x2 - 9?

    Easy
    • Ax = 3 only
    • Bx = \pm 3
    • Cx = \pm 9
    • Dx = 0
  6. 6.The roots of a quadratic are x = 1 and x = 5. What is the line of symmetry?

    Easy
    • Ax = 2
    • Bx = 3
    • Cx = 4
    • Dx = 6
  7. 7.The roots of a quadratic are x = -2 and x = 6. What is the line of symmetry?

    Easy
    • Ax = 1
    • Bx = 2
    • Cx = 4
    • Dx = 3
  8. 8.What is the line of symmetry of y = x2 + 6x + 1?

    Easy
    • Ax = 3
    • Bx = -3
    • Cx = 6
    • Dx = -6

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