单项选择(2003年全国旧课程

已知x∈(-π/2,0),cosx=4/5,则tan2x=【 】

A、7/24

B、-7/24

C、24/7

D、-24/7

答案解析

D

【解析】

∵x∈(-π/2,0),cosx=4/5,

∴sinx=-=-3/5,tanx=sinx/cosx=-3/4,

∴tan2x=2tanx/(1-tan2⁡x )=-24/7.

讨论

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