Is n negative?
(1) n^5(1 - n^4) < 0. For convenience rewrite this as n^5(n^4-1) > 0 --> n can be positive as well as negative: consider n=2 and n=-1/2. Not sufficient.
(2) n^4 - 1 < 0 --> n^4<1 --> -1<n<1. Not sufficient.
(1)+(2) Since from (2) n^4 - 1 < 0, then from (1) n^5 must also be negative (in order n^5(n^4-1) to be positive). n^5<0 means that n is negative. Sufficient.
Answer: C.
Hope it's clear.
(1) n^5(1 - n^4) < 0. For convenience rewrite this as n^5(n^4-1) > 0 --> n can be positive as well as negative: consider n=2 and n=-1/2. Not sufficient.
(2) n^4 - 1 < 0 --> n^4<1 --> -1<n<1. Not sufficient.
(1)+(2) Since from (2) n^4 - 1 < 0, then from (1) n^5 must also be negative (in order n^5(n^4-1) to be positive). n^5<0 means that n is negative. Sufficient.
Answer: C.
Hope it's clear.