单项选择题
A. B. C. D.
A.1-(1/2)cosy B.1-(1/2)cosx C.2/(2-cosy) D.2/(2-cosx)
A.4/(1-2x)2+1/x2 B.-2/(1-2x)2+1/x2 C.2/(1-2x)2-1/x2 D.-4/(1-2x)2-1/x2