Mathematics

Problem

Let

r

and

k

be positive integers with

1\le k\le r

. For

f(x)=x^{r}

, which expression correctly gives the

k

th derivative

f^{(k)}(x)

? a)

r!\,x^{r-k}

\dfrac{r!}{(r-k)!}\,x^{r-k}

r^{k}x^{r-k}

\dfrac{r!}{k!}\,x^{r-k}