题目

已知函数f(x)=x3+bx2+cx+d在区间［-1,2］上是减函数,那么b+cA.有最大值                              B.有最大值-C.有最小值                           D.有最小值- 答案：B解析:f′(x)=3x2+2bx+c,∴∴相加得b+c≤-.∴选B.I usually play football ____my friends. （ ）A. withB. andC. but