If A={x:x=3n,nϵZ} and B={x:x=4n,nϵZ} then find A∩B.
We have
A={x:xϵZand x is a multiple of 3}
B={x:xϵZ and x is a multiple of 4}
∴ A∩B=A={x:xϵZ and x is a multiple of both 3 and 4}
={x:xϵZ and x is a multiple of 12}={x:x=12n,nϵZ}
Hence, A∩B={x:x=12n,nϵZ}