I tried to use the first derivative test. d/dx n^(-1/4) = -1/4 * n^(-5/4)

-1/4 * n^(-5/4) = 0

n => does not exist

If it equalled to 0 for exampel then I could show that for n>0, the output is negative but in this case, what can I do?

Any input would be greatly appreciated!

Thanks in advance!