Why do we write the derivative of
$$\frac{d}{dK_2} max_N (A_2K_2^aN_2^{1-a} - W_2N_2)$$
as
$$\frac{d}{dK_2} A_2K_2^aN_2^{1-a} - W_2N_2 + \frac{d}{dN_2}\left ((A_2K_2^aN_2^{1-a} - W_2N_2) (\frac{dN_2}{dK_2})\right)$$
with the chain rule?
P.S the N next to max is the just $$N_2$$