KöMaL 2018 C1495

Consider the following sequence of equalities: $$\begin{align*} 1 + 2 & = 3, \tag{1}\\ 4 + 5 + 6 & = 7 + 8, \tag{2}\\ 9 + 10 + 11 + 12 & = 13 + 14 + 15. \tag{3} \end{align*}$$ By observing the rule, state the $k$th row, and prove your statement.