when you solve an equation the quadratic of g(f) must be treated like any other mathematical quantity— they must be multiplied, divided, raised to powers, cancelled, etc in exactly the same way as the numbers to which they belong
If the two waves combine to produce ANY wave that smaller
than either of the originals, that's destructive interference.
We see black colour in absence of light. Black colour absoorbs all the light, ( just opposite of white) and hence we do not see any colour. Black colour is also related to darkness.
20 ohms in parallel with 16 ohm= 8.89
20x16/20+16. Product over sum