∫﹙0-->﹢∞﹚te^(-t/2)dt
=-2∫﹙0-->﹢∞﹚te^(-t/2)d(-t/2)
=-2∫﹙0-->﹢∞﹚td(e^(-t/2))
=-2te^(-t/2)+2∫﹙0-->﹢∞﹚e^(-t/2)dt
=-2te^(-t/2)-4e^(-t/2) ﹙0-->﹢∞﹚
=4