All the bars in the illustrated square planar structure have unit stiffness.

(a) Write down the reduced incidence matrix A.
(b) Write down the equilibrium equations for the structure when subjected to external forces at the free nodes.
(c) Is the structure stable? statically determinate? Explain in detail.
(d) Find a set of external forces with the property that the upper left node moves horizontally horizontally, while the upper right node stays in place. Which bar is under the most stress?

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