Coordinate Geometry

Master the intersection of Algebra and Geometry: Lines, Slopes, and Shapes on the Cartesian Plane.

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1. The Cartesian Plane & Distance

Quadrants:

  • Q1 (+, +): Top Right
  • Q2 (-, +): Top Left
  • Q3 (-, -): Bottom Left
  • Q4 (+, -): Bottom Right

Distance Formula

Distance between A(x1,y1)A(x_1, y_1) and B(x2,y2)B(x_2, y_2):

D=(x2x1)2+(y2y1)2D = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}

2. Section Formula & Centroid

Internal Division (Ratio m:n)

P(x,y)=(mx2+nx1m+n,my2+ny1m+n)P(x, y) = \left( \frac{mx_2 + nx_1}{m+n}, \frac{my_2 + ny_1}{m+n} \right)

Midpoint (Ratio 1:1)

M(x,y)=(x1+x22,y1+y22)M(x, y) = \left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right)

Centroid of Triangle

Intersection of Medians:

G(x,y)=(x1+x2+x33,y1+y2+y33)G(x, y) = \left( \frac{x_1+x_2+x_3}{3}, \frac{y_1+y_2+y_3}{3} \right)

3. Area of Triangle (Shoelace Method)

For vertices (x1,y1),(x2,y2),(x3,y3)(x_1, y_1), (x_2, y_2), (x_3, y_3):

Area = 12x1(y2y3)+x2(y3y1)+x3(y1y2)\frac{1}{2} | x_1(y_2 - y_3) + x_2(y_3 - y_1) + x_3(y_1 - y_2) |

Note: If Area = 0, points are Collinear (Lie on the same line).

4. Slope & Equation of Line

  • Slope (mm): m=tanθ=y2y1x2x1m = \tan \theta = \frac{y_2 - y_1}{x_2 - x_1}
  • Equation (Slope-Point): yy1=m(xx1)y - y_1 = m(x - x_1)
  • Standard Form: ax+by+c=0ax + by + c = 0 \to Slope =a/b= -a/b
Parallel Lines
m1=m2m_1 = m_2
Perpendicular
m1×m2=1m_1 \times m_2 = -1
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