Derivation of beat frequency formula | Physics | Khan Academy

In this video David derives the formula for beat frequency. Created by David SantoPietro.

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[toomanycharacter]

Here's a derivation I found easier:
The waves start in phase, but then wave 2 starts lagging behind wave 1. For that to happen, wave 2 must complete more cycles than wave 1 in the same amount of time.
c1 = t*f1 = how many cycles wave 1 has completed after t seconds
c2 = t*f2 = how many cycles wave 2 has completed after t seconds
At some point, wave 2 will have completed exactly one cycle more than wave 1, which also means that wave 2 and wave 1 are perfectly aligned again! That is, at some t, c2 - c1 = 1, and that t is the amount of seconds it takes for the waves to become perfectly aligned again, so it's the period of the beat. The frequency of the beat = f = 1/t, so let's solve for f
c2 - c1 = 1
Substitute
t*f2 - t*f1 = 1
t(f2 - f1) = 1
t = 1/(f2 - f1)
1/t = f2 - f1
f = f2 - f1

[NgocNguyen-zqnx]

In my interference lesson at school I have a formula like this:
*uM = 2Acos[π(d2-d1)/λ + Δ φ/2] cos [ωt - π(d1+d2)/λ + (φ1+ φ2)/2]*

Where do I have to apply this and in which lesson on Khan Academy can I learn about this?
Thank you in advance. This one really challenged me.

[CristianoRonaldo-jj]

That's really some cool stuff. Awesome explanation

[MenShoe-ktbc]

Thanks sir am watching from Uganda, , , , yov made me understand more

[roseb]

would you care to do a video on sonic boom?

[angies]

Amazing explanation man. I'm astonished

[mshioty]

How to calculate beat frequency F3 to A3 ? (F3=174.61Hz : A3=220Hz)

[catastrophe]

Thank u so much <3 this helps in my final exams :) I needed this lecture

[akshayam]

Thanks for this wonderful explanation :)