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
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
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
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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1.What are the coordinates of the origin?
Easy- A(1, 1)
- B(0, 0)
- C(0, 1)
- D(1, 0)
2.What is the gradient of y = 4x - 1?
Easy- A-1
- B4
- C1/4
- D3
3.Where does y = 3x + 7 cross the y-axis?
Easy- A(7, 0)
- B(0, 7)
- C(0, 3)
- D(3, 7)
4.Which line is horizontal?
Easy- Ax = 4
- By = 4
- Cy = 4x
- Dy = x + 4
5.Which line is vertical?
Easy- Ay = 3
- Bx = 3
- Cy = 3x
- Dy = x
6.Find the gradient of the line through (0, 1) and (4, 9).
Easy- A2
- B4
- C1/2
- D8
7.Find the gradient of the line through (2, 7) and (5, 1).
Easy- A2
- B-2
- C1/2
- D-1/2
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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