The Statement, Split into Hypotheses and Conclusion
Figure 1 — The secant through the endpoints and a parallel tangent somewhere inside. The theorem guarantees that the parallel tangent exists; it says nothing about how many such points there are or where they sit.
Let satisfy two conditions: continuous on , differentiable on . Then there is at least one in with
The right side is the average rate of change, which is the slope of the secant line. The left side is the slope of the tangent at . The theorem says those two numbers coincide somewhere strictly inside the interval, never at an endpoint.