Grade 11 Mathematics
Grade 11 ยท Term 2Mathematics

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.

Week 1

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
๐ŸŒ

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

x2+y2=r2x^2 + y^2 = r^2

$r$ = radius; every point $(x,y)$ satisfies this

Circle centred at $(a,b)$

(xโˆ’a)2+(yโˆ’b)2=r2(x-a)^2 + (y-b)^2 = r^2

Centre $(a,b)$, radius $r$

Property / Rule

Tangent to a Circle

A tangent at point P(x1,y1)P(x_1, y_1) on the circle is PERPENDICULAR to the radius at PP. To find it: (1) find gradient of radius OPOP, (2) gradient of tangent = negative reciprocal, (3) use point-gradient form.

mradiusโ‹…mtangent=โˆ’1m_{\text{radius}} \cdot m_{\text{tangent}} = -1

๐Ÿ’ก 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 Examples

Worked Example

Find equation and tangent

Problem

Circle has centre (2,โˆ’3)(2,-3) and passes through (6,0)(6,0). Find its equation and the tangent at (6,0)(6,0).
Activity โ€” 8 Questions

CAPS Cognitive Level Distribution

L1 ยท Knowledge2 Q
L2 ยท Routine Procedures2 Q
L3 ยท Complex Procedures2 Q
L4 ยท Problem Solving2 Q
1
L1 ยท Knowledge1 mark
Write the equation of a circle with centre (0,0)(0,0) and radius 7.
2
L1 ยท Knowledge2 marks
State the centre and radius of (xโˆ’3)2+(y+1)2=16(x-3)^2+(y+1)^2=16.
3
L2 ยท Routine Procedures2 marks
Does the point (3,4)(3,4) lie on x2+y2=25x^2+y^2=25?
4
L2 ยท Routine Procedures2 marks
Find the equation of the circle with centre (โˆ’2,4)(-2,4) and radius 10\sqrt{10}.
5
L3 ยท Complex Procedures4 marks
Find the equation of the tangent to x2+y2=50x^2+y^2=50 at P(5,5)P(5,5).
6
L3 ยท Complex Procedures4 marks
Rewrite x2+6x+y2โˆ’4y=3x^2+6x+y^2-4y=3 in standard circle form and state the centre and radius.
7
L4 ยท Problem Solving4 marks
Find the length of the tangent from external point (8,0)(8,0) to the circle x2+y2=16x^2+y^2=16.
8
L4 ยท Problem Solving5 marks
Circle C1C_1: x2+y2=25x^2+y^2=25 and C2C_2: (xโˆ’8)2+y2=9(x-8)^2+y^2=9. How many points do they share?
Analytical Geometry Grade 11 Maths CAPS Notes & Examples | MathSciBuddy