2.48. Is it true that {ax + by + cz|x, y, z ∈ Z} = {n · gcd(a,
b, c)|n ∈ Z}?
2.49. What are all the integer values of e for which the
Diophantine equation 18x + 14y + 63z = e has an integer solution.
Find a solution for each such e.
2.50. For integers a, b and k > 0, is it true that a | b iff
a^k | b^k ?