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Answer» Three hinged arches:
there are three HINGES in the arch, A, B and C. Generally there are three numbers of equilibrium equation, but the fourth equation is derived from the fact the ALGEBRAIC sum of all the moments at the HING C is 0. So there are four numbers of equilibrium equations, and we can determine all the four support REACTIONS, Va, Vb, Ha, and Hb.
TWO hinged arches:
there are two hinges A and B, and there are four support reactions. There are only three numbers of equations of equilibrium, so two hinged arches are indeterminate to the degree equal to 1.
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