I am sure this is an easy question but not sure where to start:
Currencies A and B have correlation of 0.8 and currencies B and C have correlation of 0.9, what is the least correlation between A and C? The hint says write down the 3x3 correlation and denote x for the correlation bewteen A and C then only the unknown term in \Sigma is \Sigma_{13}=\Sigma_{31}. Since \Sigma is a correltation matrix it is symmetric and positive definite. So determine the interval (x_0, x_1) so that \Sigma has eigenvalues >0 provded x\epsilon (x_0,x_1). So apparently the answer is x_0. Can anyone shed any light on this question, I'm going round in circles but started with this: is this even right?

\left(\begin{array}{ccc}1&0.8&x\\0.8&1&0.9\\x&0.9&  1\end{array}\right)