[0,+∞] ∫ (2x/θ)*e^(-2x/θ)dx
=[0,+∞] ∫ (-x)*e^(-2x/θ)d(-2x/θ)
=[0,+∞] ∫ (-x)d[e^(-2x/θ)]
=[0,+∞] (-x)[e^(-2x/θ)] - ∫e^(-2x/θ)d(-x)
=[0,+∞] (-x)[e^(-2x/θ)] - θ/2 ∫e^(-2x/θ)d(-2x/θ)
= (-x)[e^(-2x/θ)] - θ/2 * e^(-2x/θ) | [0,+∞]
=θ/2谢谢，不过“[0,+∞] (-x)[e^(-2x/θ)]”为什么等于“(-x)[e^(-2x/θ)] ”呢？=[0,+∞] { (-x)[e^(-2x/θ)] - θ/2 ∫e^(-2x/θ)d(-2x/θ)}= { (-x)[e^(-2x/θ)] - θ/2 * e^(-2x/θ) } | [0,+∞]是个整体，少写了一对大括号