Question

2. Prove that the following sets are denumerable.
a. 3N, the positive integer ultiples of 3.
b. 3Z, the integer multiples of 3
c. {x: x element N and n<-12}
d. {(x,y) element N x R: xy=1}
5. {x element Z: x=1(mod5)}

Answer 1

Cay you find a bijection between the naturals and 3*naturals?

Answer 2

natural numbers are {1,2,3,4,...}
3*natural numbers are {1(3)=3,2(3)=6,3(3)=9,3(4)=12,...}

Answer 3

Find a function between those two sets then prove the function is bijective.
This one should be pretty obvious.