Imagine you're telling a friend where you left your book in a library. You don't say "near the window" — you say "third shelf, second row, fourth book from the left." You're using numbers to pin down an exact location.
Coordinate geometry does the same thing, but for points on a flat surface. It gives every point a precise address — a pair of numbers — so we can describe shapes, distances, and positions using algebra.
The Big Idea
Before coordinate geometry, geometry was about drawing shapes and proving things with logic alone. Algebra was about numbers and equations. These two worlds seemed separate.
Then René Descartes (a French mathematician) had a simple but revolutionary idea: draw two perpendicular number lines that cross at zero. Now every point on the plane has a unique pair of numbers — its coordinates.
That's it. That's the entire foundation.
The Coordinate System
Take a horizontal line — call it the x-axis. Take a vertical line — call it the y-axis. They cross at a point called the origin, labelled O.
Any point P is located by two numbers:
Its x-coordinate: how far right (positive) or left (negative) from the origin
Its y-coordinate: how far up (positive) or down (negative) from the origin
We write this as an ordered pair: (x,y).
Note
The order matters. (3,5) is not the same point as (5,3). The first number is always the horizontal position; the second is always the vertical.
A Concrete Example
Plot the point A(2,3):
Start at the origin (0,0).
Move 2 units to the right along the x-axis.
From there, move 3 units up (parallel to the y-axis).
Mark the point.
Now plot B(−1,4):
Start at the origin.
Move 1 unit left (negative x-direction).
Move 4 units up.
Mark the point.
Every point on the plane has exactly one such address. And every pair of numbers corresponds to exactly one point. This one-to-one matching is what makes coordinate geometry powerful.
The Four Quadrants
The axes divide the plane into four regions, called quadrants:
Quadrant
x-sign
y-sign
Example
I
+
+
(2,3)
II
−
+
(−1,4)
III
−
−
(−3,−2)
IV
+
−
(5,−1)
Points on the axes themselves (where either coordinate is zero) don't belong to any quadrant.
Why This Matters
Once every point has a number address, we can:
Calculate distances between points using the Pythagorean theorem
Find midpoints by averaging coordinates
Describe lines with equations like y=mx+c
Solve geometric problems using algebra instead of drawing
Important
The distance between two points (x1,y1) and (x2,y2) is:
d=(x2−x1)2+(y2−y1)2
This is just the Pythagorean theorem in disguise.
The Precise Statement
Coordinate geometry (also called analytic geometry) is the study of geometry using a coordinate system. It establishes a correspondence between:
Points on a plane and ordered pairs of real numbers
Geometric figures (lines, circles, curves) and algebraic equations …
Concept: Coordinate Geometry – Centroid via Midpoints
The centroid of a triangle divides each median in the ratio 2:1 from vertex to midpoint. A key property: the centroid is also the centroid of the triangle formed by the three midpoints of the sides.
If D, E, F are the midpoints of the sides of triangle ABC, then the centroid G of the original triangle ABC equals the centroid of triangle DEF.
The centroid of any triangle with vertices (x1,y1,z1), (x2,y2,z2), (x3,y3,z3) is:
The centroid of a triangle equals the centroid of the triangle formed by its midpoints; average the coordinates of D, E, and F to get (1,1,−2).
Why this works
When you're given the midpoints of a triangle's sides rather than its vertices, you might think you need to first recover the original vertices. But here's the beautiful fact: the centroid of the medial triangle (formed by joining the midpoints) is exactly the same as the centroid of the original triangle.
This happens because the medial triangle is similar to the original triangle and positioned such that both share the same center of mass. The centroid divides each median in the ratio 2:1 from vertex to midpoint, and this geometric relationship is preserved under the midpoint transformation.
So we can find the centroid directly from D, E, and F by averaging their coordinates.
Solution
The centroid G of any triangle with vertices (x1,y1,z1), (x2,y2,z2), and (x3,y3,z3) is given by:
G=(3x1+x2+x3,3y1+y2+y3,3z1+z2+z3)
Since D, E, and F are the midpoints forming the medial triangle, we apply this formula to their coordinates.