TeX source:

\begin{align*} F_4 &= F_2 + F_3\\ &= (F_0 + F_1) + (F_1 + F_2) \\ &= (F_0 + F_1) + (F_1 + (F_0 + F_1)) \\ &= (1+1) + (1+(1+1)) \\ &= 5 \end{align*}