I feel dumb for asking this but, how do I determine whether a certain triangle is a 3/4/5 or a 5/12/13? Im looking at an example problem in my Statics hw where they have to determine the force in each member of a truss using the method of joints and sometimes it uses the 3/4/5 rule or the 5/12/13. Can anyone help me out please?

Well, the force vector must make a 3/4/5 or a 5/12/13 triangle. the angles must match a 3/4/5 or a 5/12/13 triangle's angles.
The same concept can be used for all right triangles , 3/4/5 is just an easy/common one
So, if you know your force(F), you can find the x and y component of that force by simply using (3/5)*F or 4(5)*F. which being which (x being 3/5 or y being 3/5) is dependent on orientation of the force in the problem

This post was edited by saber_x3 on Nov 19 2013 02:09am

You don't necessarily need to know that it's a special triangle, but you must know two lengths or one length and an angle to justify the whole triangle.