Analytical Geometry
We extend Grade 10 analytical geometry to the equation of a circle centred at the origin and at any point, and find tangent lines to circles.
4.1 Equation of a Circle & Tangent Lines
- Write and use the equation of a circle $x^2+y^2=r^2$
- Write and use $(x-a)^2+(y-b)^2=r^2$ (circle with centre $(a,b)$)
- Find the equation of a tangent to a circle at a given point
Real-World Connection
Satellite orbits are circular (or elliptical). The equation of a circle precisely describes every point at distance $r$ from the centre โ a GPS satellite at orbit radius $r$ satisfies exactly this equation in a 2D cross-section.
Circle centred at origin
$r$ = radius; every point $(x,y)$ satisfies this
Circle centred at $(a,b)$
Centre $(a,b)$, radius $r$
Property / Rule
Tangent to a Circle
A tangent at point on the circle is PERPENDICULAR to the radius at . To find it: (1) find gradient of radius , (2) gradient of tangent = negative reciprocal, (3) use point-gradient form.
๐ก Tip
To check if a point lies ON the circle, substitute its coordinates into the circle equation. If LHS = RHS, the point is on the circle.
Worked Example
Find equation and tangent
Problem
CAPS Cognitive Level Distribution