How do you prove that speed equals wavelength times frequency?

Answer 1

Frequency is the number of waves per second, i.e. 20Hz is 20 waves per second.

Wavelength is obviously the length of each wave i.e 20m

So if there are 20 waves per second, the distance it travels is the length of the wave multiplied by the number of waves in one second. The distance travelled in one second by the wave above is therefore 20 * 20m wich is 400m. The distance travelled in 1 second is 400m. Therefore every second, the wave travels 400m and thus the speed is 400meters per second (400m/s).