Is zw positive?

(1) z + w3= 20

(2) z is positive.

Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

EACH statement ALONE is sufficient.

Statements (1) and (2) TOGETHER are NOT sufficient.

考题讲解

zw为正，则需要z和w同时为正或者同时为负。

（1）    如果w=-2，则z=28，满足z + ~$w^3$~ = 20，但-2*28<0。

（2）  z为正数，如果w为负，则zw<0。