pgclt

Both formulars are correct.

1) T=AUBUC+（非A非B非C的元素个数）=A+B+C-(AnB)-(AnC)-(BnC)+(AnBnC) +（非A非B非C的元素个数）
------------------------------------
(AnB), (AnC), (BnC) 求有且只有2种

2) T=AUBUC+（非A非B非C的元素个数）=A+B+C-(AnB)-(AnC)-(BnC)-2(AnBnC) +（非A非B非C的元素个数）
-----------------------------------------
(AnB), (AnC), (BnC) 求有2种or3种(AnBnC).

pgclt

You are welcome.  It's easier to see if you draw a graph with 3 circles. Good luck with your exam!

pgclt

maryland, I was wrong about the two formulars.  the conditions should reverse. hope it didn't confuse you more. thx jupiter for his correction. sorry!

1) T=AUBUC+（非A非B非C的元素个数）=A+B+C-(AnB)-(AnC)-(BnC)+(AnBnC) +（非A非B非C的元素个数）
------------------------------------
(AnB), (AnC), (BnC) 有2种or3种(AnBnC).

2) T=AUBUC+（非A非B非C的元素个数）=A+B+C-(AnB)-(AnC)-(BnC)-2(AnBnC) +（非A非B非C的元素个数）
-----------------------------------------
(AnB), (AnC), (BnC) 有且只有2种