Coordinate Geometry
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Coordinates
- The Cartesian plane has a horizontal x-axis and vertical y-axis meeting at the origin (0,0).
- Coordinates are written as (x, y) – x is horizontal, y is vertical.
- Positive x: right of origin; negative x: left. Positive y: above; negative y: below.
- Mnemonic: "Along the corridor, up the stairs" (x then y).
- Check the scale on axes – 1 square may not equal 1 unit.
Points on the Cartesian plane
Midpoint of a Line
- The midpoint is the average (mean) of the endpoints' coordinates.
- Formula: midpoint = ( (x₁+x₂)/2 , (y₁+y₂)/2 ).
- Example: A(-4,3) and B(8,-12) → midpoint = (2, -4.5).
Midpoint of AB
Gradient of a Line
- Gradient measures steepness: rise/run (change in y over change in x).
- Positive gradient: line goes uphill (bottom-left to top-right).
- Negative gradient: line goes downhill (top-left to bottom-right).
- Formula: gradient m = (y₂ - y₁)/(x₂ - x₁).
- To draw a gradient, e.g. 2/3: move 3 right, 2 up. For -5: move 1 right, 5 down.
- A gradient of 3 is steeper than 2; -5 is steeper than -4.
Gradient as rise over run
Length of a Line
- Distance between two points uses Pythagoras' theorem.
- Formula: d = √[(x₁ - x₂)² + (y₁ - y₂)²].
- Be careful with negative coordinates – use brackets to avoid errors.
- Example: A(3,-4) and B(-5,2) → d = √(8² + (-6)²) = √100 = 10 units.
Length of AB
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Câu hỏi luyện tập
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1.What is the midpoint of the line segment joining A(1, 3) and B(5, 8)?
Easy- A(3, 5.5)
- B(2, 2.5)
- C(6, 11)
- D(4, 5)
2.The point A has coordinates (5, −4). The point B has coordinates (13, 1). Work out the coordinates of the midpoint of AB.
Easy- A(9, -1.5)
- B(8, -3)
- C(18, -3)
- D(9, 2.5)
3.Find the gradient of the straight line with equation 5x + 2y = 7.
Medium- A-5/2
- B5/2
- C-2/5
- D2/5
4.Line L has equation y = 2 − 3x. Write down the gradient of line L.
Easy- A-3
- B3
- C2
- D-2
5.Find the mid-point of AB where A = (w, r) and B = (3w, t). Give your answer in its simplest form in terms of w, r and t.
Medium- A(2w, (r+t)/2)
- B(4w, r+t)
- C(2w, r+t)
- D(w, (r+t)/2)
6.A is the point (2, 3) and B is the point (7, −5). Find the coordinates of the midpoint of AB.
Easy- A(4.5, -1)
- B(9, -2)
- C(4.5, 1)
- D(5, -2)
7.Point A has coordinates (5, 8) and point B has coordinates (9, −4). Work out the gradient of AB.
Easy- A-3
- B3
- C-1/3
- D1/3
8.A straight line joins the points (3k, 6) and (k, −5). The line has a gradient of 2. Find the value of k.
Medium- A11/2
- B11/4
- C11
- D-11/2
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