Please help.. thank you for your assistance :)

Determine whether the set of whole numbers is closed under addition. explain why the set is or is not closed. give an example.

Solve the triangle. a = 21°, c = 105°, c = 5

This is a rational expression because the denominator contains a variable. this is a polynomial with 3 terms. this is a rational expression because the denominator contains a variable. this is a polynomial with 4 terms. this is a rational expression because the denominator contains a variable. this is a polynomial with 4 terms. this is a rational expression because the denominator contains a variable. this is a polynomial with 3 terms. this is a rational expression because the denominator contains a variable. this is a polynomial with 5 terms.