kmf229

(match+box)^2 = match^2 + box^2 + 2 match box
now match=w1*sd1
box=w2*sd2
and the 3rd part you are so kindly talking about refers to 2 * match * box * rho.
if rho = 1 - variance becomes (match + box)^2
if rho=0 - variance becomes match^2 + box^2
now figure…
and also closely see the brackets = the way you are thinking about it - and how ohai above has correctly explained it.