So $X_1$ and $X_2$ are two independent uniform random variables and I am looking for the PDF of $Y,$ where $Y = |X_1 − X_2|.$ I have to find the CDF of $Z = X_1 - X_2$ first, then find the CDF of $Y = |Z|,$ and then finally take the derivative to find the PDF of $Y.$. I know this:
$$F_Y(y) = P(Y ≤ y) = P(X_1 − X_2 ≤ y).$$
I found the answer to this first part here: http://www.math.wm.edu/~leemis/chart/UDR/PDFs/StandarduniformStandardtriangular.pdf but I don't understand their bounds of integration.
I know this is a long question but any help would be appreciated. I've been staring at this problem for hours. A very detailed explanation would really help me. Apologies for not understanding how to type with MathJax.