Question

Help with small arithmetic. (Calculus)

Answer 1

I understand everything in this problem, but how did the sign changed from a minus sign to plus sign?

Answer 2

Think of it instead as multiplying top and bottom by 1/sqrt(x^2).

That should go straight into the radical but for the x terms note that 1/sqrt(x^2) = 1/|x| = -1/x as x<0

Different algebra but same result.