Past exam of the short options course of the University of Oxford / 2021 / 1 / a / i / Solution

No. Take $P=2$, $Q=9$. The Jacobi symbol isbut the squares modulo $3$ are $0,1$. A square root of $2$ modulo $9$ would reduce to one modulo $3$, which is impossible. A Jacobi symbol of $1$ records a product of local signs; it does not assert that every local sign is positive, or that a quadratic congruence is soluble.