TeX source:

\tilde{s} = (\tilde{x}_1+\tilde{x}_2)(1+\epsilon_3 \\ =x_1(1+\epsilon_1)(1+\epsilon_3) + x_2(1+\epsilon_2)(1+\epsilon_3) \\ \approx x_1 (1+\epsilon_1+\epsilon_3) + x_2(1+\epsilon_1+\epsilon_3) \\ \approx s(1+2\epsilon)