show that $4k+2$ is not a square of a natural number
I don't know how to start with this problem.
I thought that maybe $(4k+2)^2=4(4k^2+4k+1)$ help me but I don't know what to do next.
$4k+2 = 2(2k+1)$. It can't be a square because it is divisible by $2$ but not by $4$ ($2k+1$ is odd).
Hope that helps,