There are only two things we need to remember about the tangent line to f at a.
- The tangent line and f have the same y-value at a.
That is, the point (a, f(a)) is on f and also on the tangent line to f at a.
- The tangent line and f have the same slope at a.
That is, the slope of the tangent line to f at a is f'(a).
That's it. If we can we remember those two things we'll be good when it comes to tangent lines.