Identify the correct statements concerning relativistic and...

The equations of special relativity are fundamental and apply to all speeds $v < c$. Therefore, they are valid across all speeds.

Nonrelativistic equations are approximations of relativistic equations obtained by taking the limit $v \ll c$. Since they are approximations, they never produce exact results for any non-zero speed, although the deviation from exact results is negligible at small speeds.

Thus, the correct statements are that nonrelativistic equations never produce exact results and the equations of special relativity are valid across all speeds.