Hello, DIOGYK!

I think I got it . . .

We have: .

. . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . . . .

. .

n .

. . . . . . .

Factor: . . . . .

. . . . . . . . . . .

. . . . . . . . . .

Factor: . . . . . . . . .

We see that the product of three factors equals zero.

Hence, at least one of the factors must be zero.

Therefore: .