Chapter 8 - Complex Numbers, Polar Equations, and Parametric Equations - Section 8.1 Complex Numbers - 8.1 Exercises - Page 357: 37

-13

$\sqrt -13\times\sqrt -13=\sqrt (-1\times13)\times\sqrt (-1\times13)=\sqrt -1\times\sqrt 13\times\sqrt -1\times\sqrt 13=i\times i\times\sqrt 13\times\sqrt 13=i^{2}\times(\sqrt 13)^{2}=-1\times13=-13$ We know that $\sqrt -1=i$, because $i^{2}=-1$.