Drag point A around the curve to see how the tangent changes.
To enable trace, right click point B and click to turn trace on.

What is the equation of the curve? How do you know?
What is the relationship between the gradient of the tangent at point A and point B?
What is the relationship between point A and point B?
On point B, turn trace on. Drag point A around. What do you notice?
What is the equation of the line drawn by point B?
Can you think of any relationship between the equation of the curve and the equation of the line?