Fundamental frequency
A signal which is the sum of sinusoids with different frequencies $f_i$ is only periodic if

$$f_i=k_if_0,\quad i=1,2,\ldots$$

is satisfied for some $f_0$ and integer values $k_i$. Or, in other words, the frequencies $f_i$ must be rational multiples of each other. In your example, the signal is periodic but its fundamental frequency is not $f_1=800\,\text{Hz}$ because $f_2\neq k_2f_1$ for integer $k_2$. The fundamental frequency is $f_0=50\,\text{Hz}$, even though this frequency does not explicitly occur in the signal. However, note that its period equals $1/f_0$.