If X~Exp(a), Y~Exp(b), consider (X|X>Y). My book said that (X|X>Y)=(X-Y+Y|X>Y)=(X-Y|X>Y)+(Y|X>Y). But Why?? I don't know why we can divide those two things.
Stack Overflow doesn't support MathJax/LaTeX to my knowledge so here is an image: