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Coordinates and straight-line graphs

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Coordinates and the gradient

  • Points are written (x, y): across first, then up or down. The axes cross at the origin (0, 0).
  • The gradient measures steepness: m = \frac{\text{change in } y}{\text{change in } x} = \frac{y2 - y1}{x2 - x1}.
  • A line sloping up from left to right has positive gradient; sloping down, negative; horizontal, zero.

Gradient from two points

Gradient from two pointsO−4−224681012−11234xyy = 3x − 1gradient= 6 / 2 =3

The equation \(y = mx + c\)

  • Every non-vertical straight line can be written as y = mx + c, where m is the gradient and c is the y-intercept. A vertical line has equation x = k and undefined gradient.
  • y = 2x + 3 has gradient 2 and crosses the y-axis at (0, 3).
  • Horizontal lines are y = k; vertical lines are x = k.

Reading y = 2x + 3

Reading y = 2x + 3O−4−224681012−3−2−11234xy(0, 3)gradient= 2 / 1 =2y = 2x + 3

Drawing a line

  • Make a table of values: substitute at least three x-values and plot the points.
  • Or use the gradient and intercept: start at c on the y-axis, then move across 1 and up m.

Finding the equation of a line

  • Through two points: work out the gradient m, then substitute one point into y = mx + c to find c.
  • Through (1, 5) and (3, 9): m = \frac{9 - 5}{3 - 1} = 2; 5 = 2(1) + c gives c = 3, so y = 2x + 3.

Line through two points

Line through two pointsO24681012−11234xy(1, 5)(3, 9)y = 2x + 3

Midpoints and lengths

  • The midpoint of (x1, y1) and (x2, y2) is \left(\frac{x1 + x2}{2}, \frac{y1 + y2}{2}\right).
  • The length of a line segment comes from Pythagoras' theorem.

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Übungsfragen

Gratis-Vorschau — 8 von 32 Fragen. Registriere dich, um alle zu sehen.
  1. 1.What are the coordinates of the origin?

    Easy
    • A(1, 1)
    • B(0, 0)
    • C(0, 1)
    • D(1, 0)
  2. 2.What is the gradient of y = 4x - 1?

    Easy
    • A-1
    • B4
    • C1/4
    • D3
  3. 3.Where does y = 3x + 7 cross the y-axis?

    Easy
    • A(7, 0)
    • B(0, 7)
    • C(0, 3)
    • D(3, 7)
  4. 4.Which line is horizontal?

    Easy
    • Ax = 4
    • By = 4
    • Cy = 4x
    • Dy = x + 4
  5. 5.Which line is vertical?

    Easy
    • Ay = 3
    • Bx = 3
    • Cy = 3x
    • Dy = x
  6. 6.Find the gradient of the line through (0, 1) and (4, 9).

    Easy
    • A2
    • B4
    • C1/2
    • D8
  7. 7.Find the gradient of the line through (2, 7) and (5, 1).

    Easy
    • A2
    • B-2
    • C1/2
    • D-1/2
  8. 8.Which point lies on y = 2x + 1?

    Easy
    • A(3, 6)
    • B(3, 7)
    • C(2, 6)
    • D(1, 4)

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