Assuming k is a positive integer, another rather crude way to approach it is to realise that
\dfrac{k}{(2k+1)(2k+2)}=\dfrac{1}{2}\times\dfrac{k}{(2k+1)(k+1)}
Notice that
\dfrac{k}{(2k+1)(k+1)} < 1
because each factor in the denominator is clearly larger than the numerator, hence the whole...