Low-frequency response of a CS amplifier
The statement below the red boxed text is fundamentally wrong because you cannot simply numerically add the frequencies of several cascaded filters and produce a number that has any accurate meaning. There might be some particular cases where it has some semi coincidental accuracy but these situations are not obvious and it would be naive to assume it will work. Given the usage of free sim tools I would advise against using this formula in favour of performing a simulation.