Big Long Cov(i,j) Formula--Memorize it or just the Shortened One?

I know that Cov(i,j)=Beta(i)*Beta(j)*Variance(M). I cannot memorize:


Will we have to use this longer one, or is it safe to say just shorter one-factor version?

I had to use this.

Correction: I tried to use this.

Fudge. So I guess memorize it.