Because to be this exact we would have to measure down to the size of the atom, and this isn't important/cost effective/practical in 99.999999% of the areas that we would use measurements...
Depends upon how good the measurer is at measuring.
Yes, the Pythagorean theorem gives the exact measurements always. It can be backed up by proofs and sin, cosine, etc.
metric system as it is worked on calculation of the distance of earth from the sun. and use's that as a frame of reference. Kevin
Greater than 90 but less than 180 degrees