
Step-by-step explanation
The beat frequency is equal to the complete value of the alteration in the frequency of the two waves. The count of beats per second is equivalent to the difference in frequencies of two waves is called beat frequency.
it is given by the expression:

then
Step 1
let

now,replace

Step 2
do, the same for the same pair of frequencies
let

now,replace

Step 3
the last pair of frequencies:
let

now,replace

I hope this helps you