A doubt in C3_W2 Lecture

Dear Administrator,

Could you please guide me on how to prove that the combination of Equation 1 and Equation 2 produces Equation 3?

Equation 1:

Equation 2:

Equation 3:

Thank you.

The only change is in the summation term.

This summation term is the sum of the squared difference for all possible values of i and j (movies and users).