



 
Trevor, If the building is exactly 40x30, and a rectangle, then the hypotenuse should be exactly 50. If it is not a rectangle, one diagonal will be too long, the other too short. If your quadrilateral has the right edge lengths, you can make it into a rectangle by physically shortening the long diagonal or stretching the short one. Again, if the edge lengths are right, it is necessary and sufficient (for the frame to be a rectangle) to have the diagonals equal. This can be used for a "measuringtapefree" construction using a cord to compare lengths. Good Hunting!
 


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