Add study log for 2026-03-19: Khan Academy Sets

Co-authored-by: han <han@noreply.fbrns.co>

study-log.md

@@ -20,3 +20,17 @@ Questions:
 ---
 
 **Log** (newest first)
+
+**2026-03-19**
+- Khan Academy: Sets & Their Representations (110 minutes) — https://www.khanacademy.org/math/ka-math-class-11/x0419e5b3b578592a:sets-ncert-new/x0419e5b3b578592a:sets-and-their-representations/v/what-are-sets
+
+Notes:
+- Roster form: lists all elements explicitly, e.g. {1, 2, 3, ...}
+- Set-builder form: describes elements by a rule, e.g. {x: x is an odd number less than 10}
+- Sets can be empty, finite, or infinite
+
+Questions:
+- Still unclear on terminology: positive integer, real number, integral number, etc. — these are number classification names, not strictly a set concept. Looking for a reference to solidify these.
+
+Comment:
+- Notes are solid. The number classification question is actually about "number systems" — you'll encounter these more in the lecture notes when they talk about fields and groups. For now, just know that N = natural numbers, Z = integers, R = real numbers. Shortcut: look up "number sets notation" on Khan Academy or Wikipedia. Don't spend too long here — it'll click once you see them used in context.