For the function: $f(x)=5x^3-10x$ we find the zeroes to be: $x=0, \sqrt{2}$, and $-\sqrt{2}$
However... If you let $a=\sqrt{2}$ or $-\sqrt{2}$ in $5a^3-10a$
Desmos evaluates and says either of those equal $3.5527136788*10^{-15}$ (NOT $0$)!!! Am I doing something incorrectly or is this Desmos' fault?
Image:
Desmos is not a true symbolic calculator, and the results you see are due to floating-point error incurred when the (inexact) values for $a$ are substituted into $5a^3-10a$ under (an also inexact) floating-point regime.