Give a definition for the following term. Are there other terms that need to be defined first? What are they and how might you define them?

Radius of a circle

Give a definition for the following term. Are there other terms that need to be defined first? What are they and how might you define them?

Square

Consider two 'postulates' given below:

(i) Given any two distinct points $A$ and $B$, there exists a third point $C$ which is in between $A$ and $B$.

(ii) There exist at least three points that are not on the same line.

Do these postulates contain any undefined terms? Are there postulates consistent? Do they follow from Euclid's postulates? Explain.

Does Euclid fifth postulate imply the existence of parallel lines? Explain.