Restrained Beams Deflection

I have a question regarding Problem 719. I've got the correct answer for solving for R. But when i try to solve using my own way of solving the midspan deflection, my answer is wrong. So i consult your solution. Although your answer is the same as in the textbook, I was confused why the tangent line was drawn from B which is by inspection, the tangent line drawn from a fixed end is horizontal.

Thank you for your response.

Arthur