Let the intersection point of cubic-3x of curve C: y = x and straight line x = a (a > 0) be P

Let the intersection point of cubic-3x of curve C: y = x and straight line x = a (a > 0) be P

The coordinates of the intersection point P of y = x & # 179; - 3x and x = a are p (a, a & # 179; - 3a) y '= 3x & # 178; - 3, and the tangent slope at P point is 3A & # 178; - 3. Then the tangent is Y - (A & # 179; - 3a) = (3a & # 178; - 3) (x-a) and the point Q (- A, 0)  (3a & # 178; - 3) (- A-A) = (A & # 179; - 3a)