## Calculus (3rd Edition)

$6!$
If $f(x)=x^6$, then we have $$f'(x)=6x^5, \quad f''(x)=5\times 6 x^4, \quad f'''(x)=4\times 5\times 6 x^3,$$ $$f^{(4)}(x)=3\times 4\times 5\times 6 x^2, \quad f^{(5)}(x)=2\times 3\times 4\times 5\times 6 x$$ and finally, the sixth derivative is $f^{(6)}(x)=1\times 2\times 3\times 4\times 5\times 6 =6!$.