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题库 / GMATLA-DS-23
If a and b are positive, is~$(a^{-1}+b^{-1})^{-1}$~ less than ~$(a^{-1}b^{-1})^{-1}?$~
(1) ~$a = 2b$~
(2) ~$a + b > 1$~
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.
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漏看题目条件 a b 是正数，所以B选项是 ab / (a+b) < ab, 又 a + b >1，两边正数的情况下左边除以一个大于1的数一定更小，就算ab是分数也一样
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