The first question: it is a to b, (end coordinate Y- starting coordinate Y)/ (end coordinate X- starting coordinate x), why is it defined! ! There is only one symbol difference between a and B and B to a, and the difference is exactly at which angle the arc of a symbol is tangent.

The second question: actually, it is the principle of reflection. Reflection angle = incident angle. Calculate the incident angle from B to C -A to B (let there be a point E on the extension line of CB, that is, the angle EBA is the incident angle), let there be a point P in the positive direction and a point Q in the negative direction of the tangent, and a point F on the extension line of AB makes BF=AB, 90-incident angle = angle ABP= angle DBQ=