Surprisingly, when the quadrilateral is a parallelogram, like in a scissor lift, or “cyclic”, when the endpoints lie on a circle, the bar framework can move.

Solution: All 4 vertices touch the circumference of the circle. Using theorem 4. Opposite angles in a cyclic quadrilateral add up to 180° a = 180 – 60 = 120° b = 180 – 140 = 40° ...