Question

Can someone tell me if i did this covariance question correctly?

Answer 1

Find Cov(x,y) if var(x+y)=9 and var(x-y)=6.

var(x+y)= var(x)+var(y)+2cov(x,y) = 9
var(x-y)= var(x)+var(y)-2cov(x,y) = 6

so because variance is a linear operator, i subtracted getting -4cov(x,y)=-3. which gives me cov(x,y) = 3/4