Mathematics
The Power Rule for Derivatives and Its Limitations
Quick fact
The power rule d/dx(x^n) = n·x^(n-1) holds for any real constant exponent n, but it does not apply when the exponent itself is a variable (like x^x) or when both base and exponent vary.
Why this is interesting
You've learned that the derivative of x^3 is 3x^2, but what about x^x? The simple power rule fails there—why?