Solving for F1  
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The figure shows the free body diagram of the aluminum bar sandwiched between copper bars. The free body diagram cuts across the three bars. The forces on the copper bars at the top and bottom are F subscript 1 directed rightwards. The force on the aluminum bar is F subscript 2 directed rightwards.

Substitute the results of the equilibrium equation

into the compatibility equation and solve for the axial force in copper bar (1).

Since L1 = L2, the length terms cancel out.