题目

Mr Smith_______ out. He'll be back in about an hour. A. has been     B. was           C. has left       D. has gone 答案：D已知函数φ（x）=lnx．（1）若曲线g（x）=φ（x）+ax-1在点（2，g（2））处的切线与直线3x+y-1=0平行，求a的值；（2）求证函数f（x）=φ（x）-2(x-1)x+1在（0，+∞）上为单调增函数；（3）设m，n∈R+，且m≠n，求证：m-nm+n＜|lnm-lnn2|．