Chọn A
Đặt: \(\left\{\begin{array}{l} {u=x} \\ {{\rm dv}=e^{x} {\rm dx}} \end{array}\right. \Rightarrow \left\{\begin{array}{l} {{\rm du}={\rm dx}} \\ {v=e^{x} } \end{array}\right. \)
khi đó ta có:\(\int xe^{x} {\rm dx}=e^{x} x-\int e^{x} {\rm dx}=e^{x} (x-1) +c.\)
Nên a+b=0.