What is the principal root of -25?

Answer 1

The principal root of a number is the positive real square root of the number - if it exists.

Since -25 is negative, it does not have a real square root. Its square roots are the imaginary numbers +/-5i where i is the [imaginary] square root of -1. i is neither positive nor negative and so neither +5i nor -5i is a principal root.

Answer 2

That's the positive one; the one shown on a calculator when you calculate the square root of 25.