1）Given a circle and a quadrilateral ABCD whose vertices all lie on the circle. Let R be the midpoint of are AB. The line RC meets line AB at point S, and the line RD meets line AB at point T.
Prove that CDTS is a cyclic quadrilateral.
Note : All vertices of a cyclic quadrilateral lie on a circle.
2）Given an integer n. We rearrange the digits of n to get another number m
Prove that it is impossible to get m+n = 999999999.
谢谢解答！
