Why not 2cov in this question?

Okay, So this is the whole question. It's a bit far. But i just have one question about it:

So in question c) we have the equation for the sharpe ratio as: $$\frac{E(w_1R_{1A} + w_2R_{2A})}{\sigma_c}$$

Where

$$\sigma_C = w_1^2 var(\beta_1 R_{1t}) + w_2^2 var(\beta_2 R_{2t}) + 2w_1 w_2 Cov(R_1R_2)$$

We have the $$2cov$$ here, but in e) question. The right answer is that they not have the $$2cov$$ here. Otherwise it is the same equation.

Why?

/

It’s not that you exclude it, that’s sort of misinterpretation, it is just zero ($$cov(R_1,R_2)=0 \implies 2w_1 w_2 Cov(R_1R_2) =0$$) and per convention zero is usually dropped from equations.
• @1mflon1 thank you :* But how did you see that $cov(R1, R2) =0$? – soetirl13 Nov 20 '18 at 11:40