Explanation:
At GCSE, a circle with centre (0, 0) and radius r has equation x2 + y2 = r2. Because a tangent to a circle is always perpendicular to the radius at the point of contact P(x1, y1), their gradients multiply to give −1.
Follow these 3 steps to find the equation of the tangent at P(x1, y1): (1) find the gradient of the radius OP using mr = y1x1; (2) find the perpendicular tangent gradient mt = −1mr; (3) substitute mt and (x1, y1) into y − y1 = mt(x − x1).
Example 1: Find the equation of the tangent to x2 + y2 = 25 at the point P(3, −4).
Solution:
Gradient of radius OP: mr = −4 − 03 − 0 = −43
Gradient of tangent (perpendicular): mt = +34
Equation: y − (−4) = 34(x − 3) ⇒ 4y + 16 = 3x − 9
Answer: y = 34x − 254 (or 3x − 4y = 25)
Example 2: The tangent to x2 + y2 = 20 at A(2, 4) crosses the x-axis at B. Find the coordinates of B.
Solution:
Gradient of radius OA: mr = 42 = 2 ⇒ mt = −12
Tangent: y − 4 = −12(x − 2) ⇒ y = −12x + 5
On the x-axis, y = 0: −12x + 5 = 0 ⇒ x = 10
Answer: B(10, 0)