Is cdef < 0?
c d^2 e^3 f^4 < 0
(?) (+/-) (?) (+/-) < 0
This only tell us that c and e have different sign, not sufficient.
2) c^2 d^3 e^4 f^5 > 0
(+/-) (?) (+/-) (?) > 0
This only tell us that d and f have the same sign, not sufficient.
3) Combined:
c and e have different sign, thus their product will be (negative)
d and f have the same sign, thus their product will be (positive)
(negative) * (positive) < 0
c d e f <0
Sufficient
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