Replacing "x" is not right!

[roadrunner]

Spybot,

I could be wrong, but I don't think what you used is a replacement operator. It's a constraint operator. In words you asked the calculator to tell you what x^4-x^2+5 is if x=-x. The only time x=-x is when x=0 so it stuck in 0 for x and returned 5.

You probably want subst(x^4-x^2+5,x=-x) which returns (-x)^4-(-x)^2+5.

-road