So if what you
So if I understand you correctly, what you're saying is that all trusses made of solely triangles are internally statically determinate. Then can you explain why this is (internally) statically determinate?
The truss in the third figure can be easily formed by starting with one triangle and then successively adding two members and a joint between them (first and second figures). However, I don't see how this can be done with the last figure, even if it is made of triangles.
My professor taught me that a internally statically determinate truss is formed by having one basic truss - the triangular truss consisting of three joints and three members:
and then repeatedly adding two members and a joint:
So, it makes sense why this is a statically determinate truss...