▷ input data
  - L : Length of simple beam (m)
  - P : Point load (N)
  - a : Distance from left (A) support point (m), (a≤L)
  - x : Distance from left (A) support point where the bending moment is calculated (m), (x≤L)

1. Reaction force at left support point (A)
    ∑ Moment at right support point (B) = 0, VA*L = P*(L-a)

  ∑ Vertical force = 0, P-VA-VB = 0

2. Bending moment at x

  (1) 0 < x <= a

  (2) a < x <= L