Equation of a Circle

Circle equations

• A circle of radius r centred at the origin has equation
  • x^2 + y^2 = r^2
• Be careful with the right-hand side of this equation, it is the radius squared
• A line which just touches the circle once is called a tangent to the circle
• The angle between the tangent and the radius is 90

Problem solved

• Start by drawing a sketch so you can see what is going on
• A sketch should be neat so use a ruler and compasses
• Label all the important information