Question

Would I be right if I said 21pi/10 and -pi/10 are two angles coterminal with pi/10?

Answer 1

Two angles are coterminal if they differentiate in a factor of k*(2pi). This is  a multiple of 360 degrees.

In this case 21 [pi /10] = [pi / 10] + 20 [pi/10] = [pi/10] + 2pi, then 21pi/10 is coterminal with pi/10.

In the case of -pi/10 you cannot get to pi/10 by adding a multiple of 2pi , then they are not coternminals.

Conclusion: you would not be right.