C.-                                            D.-2

24. (         )

A.                                              B.

C.                                                 D.arctgx+C

25. (         )

A.0                               B.1                               C.+                           D. 不存在

26. (         )

A.>0                             B.<0                                    C.=0                                    D. 不能确定

27. (         )

A.-                            B.                                    C.2                               D.-2