Why do this problem?
This problem involves the interpretation of a very simple concrete structure, a linkage of 4 rods, and the angles that the quadrilateral formed by the rods could make if the joints between the rods at the vertices are totally flexible. Experimental evidence will offer ideas which then need justification and proof by forming convincing arguments.

The solution uses the cosine and sine rules. To find the constraints on the angles in the general case requires an argument using inequalities.

Possible approach
You might allow time for learners to explore the quadrilateral using strips of card or plastic and split pins, or a dynamic geometry package. This will help them to identify what can be varied and what not.

Discuss the freedoms and constraints within the problem, the impact these might have and how they could influence the structure of any investigation (what can be changed and what cannot).

Encourage groups to identify ideas that they would like to investigate. Spend time planning what they might do and sharing ideas before developing them.

Share findings and approaches.

Key questions

• What are your variables?
• If you flex the quadrilateral can the angles be any size?
• Can you find a relation between the cosines of opposite angles?
• What constraints would you like to impose? For example, that the quadrilateral is cyclic.

Possible support
Try the problem Diagonals for Area, also about bendy quads but only using the area of a triangle.

Possible extension
Try Biggest Bendy , Flexi Quads , Flexi Quad Tan