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Coordinate Geometry

infoWhy this? Coordinate geometry unites number, algebra, and shape by representing positions and relationships on a plane. Accurate plotting, coordinate differences, and simple line equations build spatial reasoning and provide a foundation for graphing functions.

scheduleWhy now? Previous work on transformations has introduced movement on grids, while algebra and equations have established variables and equality. This unit brings those ideas together before later study of gradient, intercepts, and general straight-line graphs.

neurologyYou need to know

  • Coordinates are written as an ordered pair `(x, y)`, where `x` is the horizontal coordinate (abscissa) and `y` is the vertical coordinate (ordinate).
  • The x-axis is horizontal and the y-axis is vertical; they intersect at the origin `(0, 0)`.
  • In the first quadrant, both `x` and `y` are positive; in the second quadrant, `x` is negative and `y` is positive; in the third quadrant, both are negative; and in the fourth quadrant, `x` is positive and `y` is negative.
  • Moving right increases `x`; moving left decreases `x`; moving up increases `y`; and moving down decreases `y` on a Cartesian grid.
  • On the x-axis, the y-coordinate is 0, and on the y-axis, the x-coordinate is 0.
  • Axes on coordinate grids must be labelled with variable names and marked with equal scale intervals; positive and negative directions must be indicated.
  • The horizontal distance between two points with the same y-coordinate equals the absolute difference of their x-coordinates; the vertical distance between two points with the same x-coordinate equals the absolute difference of their y-coordinates.
  • A horizontal straight line is parallel to the x-axis and has equation `y = b`; every point on the line has the same y-value, `b`.
  • A vertical straight line is parallel to the y-axis and has equation `x = a`; every point on the line has the same x-value, `a`.
  • The graph of `y = x` is a straight line through the origin with gradient 1; every point on it has equal coordinates `(x, x)`.
  • The graph of `y = -x` is a straight line through the origin with gradient −1; every point on it has coordinates `(x, -x)`.
  • The lines `y = x` and `y = -x` are perpendicular, and each makes a 45° angle with the coordinate axes.
  • Reflecting a point in the y-axis changes `(x, y)` to `(-x, y)`; reflecting a point in the x-axis changes `(x, y)` to `(x, -y)`.
  • The x-intercept of a graph is the point where `y = 0`; the y-intercept is the point where `x = 0`.
  • Axis-aligned rectangles on a grid have opposite sides parallel to the axes; their vertices occur in pairs that share x-values and in pairs that share y-values.
  • An axis-aligned square has four equal side lengths; adjacent vertices differ in only one coordinate by the common side length.
  • In an axis-aligned right-angled triangle, the two legs lie on a horizontal and a vertical line, so two vertices share an x-value and two share a y-value.
  • On `y = x`, increasing `x` by 1 increases `y` by 1; on `y = -x`, increasing `x` by 1 decreases `y` by 1.

rocket_launchYou must be able to

  • Draw and label perpendicular x- and y-axes with a ruler, marking the origin and equal scale intervals, and indicating positive and negative directions clearly.
  • Plot points in the first quadrant by locating `x` along the x-axis, moving up by `y`, and marking the point accurately with a cross. Exemplification
  • Plot points in all four quadrants accurately by using the signs of `x` and `y` to choose the correct directions from the origin.
  • Read and write the coordinates of a given point from a grid in the correct order `(x, y)`, including negative values.
  • Identify the quadrant of a point and justify it using the sign pattern of its coordinates.
  • Recognise, sketch, and label the lines `x = a` and `y = b` by drawing straight vertical or horizontal lines through the correct intercepts using a straightedge.
  • Plot and label the graphs of `y = x` and `y = -x` across all four quadrants by generating at least two correct points for each and joining them with a straightedge through the origin.
  • Determine the coordinates of missing vertices of an axis-aligned rectangle or square by using shared x- or y-values and equal side lengths.
  • Use horizontal and vertical coordinate differences to calculate side lengths and verify whether a shape on the grid is a rectangle, square, or right-angled triangle.
  • Find and label the intercepts of a line by substituting `x = 0` to get the y-intercept and `y = 0` to get the x-intercept.


Revision Quiz

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