${\mathrm{\delta }}_1=\frac{{\mathrm{F}}_1{\mathrm{L}}_1}{{\mathrm{A}}_1{\mathrm{E}}_1}$

and

${\mathrm{\delta }}_2=\frac{{\mathrm{F}}_2{\mathrm{L}}_2}{{\mathrm{A}}_2{\mathrm{E}}_2}$

into the geometry of deformations equation

${\delta }_1+{\mathrm{\delta }}_2=0$

to obtain the following equation:

$\frac{{\mathrm{F}}_1{\mathrm{L}}_1}{{\mathrm{A}}_1{\mathrm{E}}_1}+\frac{{\mathrm{F}}_2{\mathrm{L}}_2}{{\mathrm{A}}_2{\mathrm{E}}_2}=0$

This equation relating ${\mathrm{F}}_1$ and ${\mathrm{F}}_2$ is called the compatibility equation.