Suppose that we have two IID random variables, $X_1, X_2$, carried by a triple $(\Omega,\mathcal{F},P)$.
While solving an exercise I ended to a point that I had to see that,
$ \iint\limits_D x_1 P_X(\mathrm dx_1)P_X(\mathrm dx_2) = \iint\limits_D x_2 P_X(\mathrm dx_2)P_X(\mathrm dx_1), $ where $D= \{ (x_1,x_2) \in \mathbb{R}^2 : x_1 + x_2 = c \} $ and $ P_X$ is the law of $X_1, X_2$, i.e. the measure induced by $P$
The only way that I could identify it was by a change of variables.
Anyone can give me some intuition about this equality? Anyone can tell me something more simple than change of variables?