code: static double adjacentLength(Vector2d hypotenuse, Vector2d adjacent, double oppositeLength) {
          return Math.tan(hypotenuse.angle(adjacent)) * oppositeLength;
      }
docs: Given vectors for the hypotenuse and adjacent side of a right angled
  triangle and the length of the opposite side, determine how long the
  adjacent side size.
  
  @param hypotenuse vector for the hypotenuse
  @param adjacent vector for the adjacent side
  @param oppositeLength length of the opposite side of a triangle
  @return length of the adjacent side