Is either of these inequalities true? $$\lambda(tA + (1t)B)\geq t\lambda(A) + (1t)\lambda(B)$$ or $$\lambda(tA + (1t)B)\leq t\lambda(A) + (1t)\lambda(B),$$ where $0\leq t \leq 1$, $A,B$ are bounded domains in $\mathbb{R}^n$ and $\lambda(\Omega)$ is an eigenvalue of the problem $$\Delta\,u=\lambda\,u\,\,\mbox{in}\,\,\, \Omega, \, u=0\,\,\,\mbox{on}\,\,\, \partial\Omega.$$
Update: I intended this to be a complete answer originally, but my "counterexample" was based on a miscalculation. So this is now more a collection of remarks on what I think the question is about.
We need to be more specific about which eigenvalue we want to take. I will discuss the ground state energy, which seems natural.
The first inequality is clearly false because $\lambda(\Omega)$ becomes large when we make the region small. As for the second, I want to rename $A'=tA$, $B'=(1t)B$ (and then drop the primes again). Then the claim becomes $$ \lambda(A+B) \le t^3\lambda (A) + (1t)^3 \lambda(B) , $$ for all $0\le t\le 1$. Minimize over $t$ to rewrite this as $$ \lambda(A+B) \le \frac{\lambda(A)\lambda(B)}{(\sqrt{\lambda (A)} + \sqrt{\lambda (B)})^2} . \quad\quad\quad\quad (1) $$ This is true in one dimension, with equality; recall that $\lambda([0,L])=\pi^2/L^2$ to see this. I don't know what happens in higher dimensions; preliminary attempts at easy counterexamples (rectangles ...) were unsuccessful, and if I had to guess, I would now say that (1) is probably true. The inequality does hold in the special case $A=B$ (because then $A+A\supseteq 2A$).

$\begingroup$ Please, Christian Remling, you can give me some reference. $\endgroup$ – de Araujo Jun 4 '15 at 22:17

$\begingroup$ @deAraujo: References on what exactly? I'm not really using anything in this answer. $\endgroup$ – Christian Remling Jun 4 '15 at 22:22

$\begingroup$ Please, you could explain to me again how it obtained the expression $\lambda(A+B)\leq t^3\lambda(A) + (1t)^3\lambda(B)$. $\endgroup$ – de Araujo Jun 4 '15 at 22:29

$\begingroup$ @deAraujo: Use that $\lambda (tC)=t^{2}\lambda (C)$, which just follows from the definitions: if $u(x)$ solves $\Delta u=\lambda u$ on $C$, then $u(x/t)$ works on $tC$ and vice versa. $\endgroup$ – Christian Remling Jun 4 '15 at 22:31

$\begingroup$ Right, note that (1) is also true, with equality, for $\lambda_k([0,L])=\pi^2k^2/L^2$, for each $k\geq 1$ integer. $\endgroup$ – de Araujo Jun 5 '15 at 0:23