Given a function f(x), and two points on the curve (a, f(a)) and (b, f(b)) the line joining these two points is called a secant line of f. (Not to be confused with the trigonometric secant (sec) ...

You need both a point and the gradient to find its equation. You are usually given the point - it's where the tangent meets the circle. To find the gradient use the fact that the tangent is ...