Hi guys, i've been struggling with a few bits and pieces from a homework sheet and have no idea how to present the following q's. I'm a final year undergrad. Can anyone provide me with solutions to the following?

1.Show that if gcd(a, c) = gcd(b, c) = 1 then gcd(ab, c) = 1

2.The equation X^2 + Y ^2 = 3Z^2 has no non trivial solution

(i.e. 6= (0, 0, 0)). Show the same for X^2 + Y ^2 = 11Z^2.

3.Find a formula giving all the solutions to the equation X^2 + 2Y^2 = Z^2 with X,

Y and Z in N and gcd(X, Y, Z) = 1.

4.Determine whether or not the following polynomials are irreducible in Q[X] X^3 +2X^2 +4X +2.