题目已知ln f(x) = sin x，则int (x cdot f'(x))/(f(x)) dx = x cos x + sin x + C（ ）A. 对B. 错已知$\ln f(x) = \sin x$，则$\int \frac{x \cdot f'(x)}{f(x)} dx = x \cos x + \sin x + C$（ ）A. 对B. 错题目解答答案B. 错