The figure shows a possible arrangement of an isochronic pendulum, that is, a pendulum that does not change in frequency with changes in ambient temperature. The three light vertical bars, of length $ L $, have coefficients of linear thermal expansion $ alpha_1 $ and the two dark vertical bars, of length $ L_2 $ have coefficient $ alpha_2 $. Determine $ L_1 $ and $ L_2 $ so that the length $ L $ remains constant. Suppose the joints and supports are fixed and rigid.
Can someone help me? I think it is possible to solve … but how? It does not make it very clear what is fixed and what is not