L2_A
# ways n people stand in a line?
Stand in a ring? (cyclic shift counted the same)
L2_B
0~〖10〗^10 How many of them contain“1”?
n-digit binary string (0,1)
Q:How many contain even # of 0s?
A:Half of them
L2_C
n-digit 4-ary string (0,1,2,3)
Q:How many contain even # of 0s?
A:2^n＋(4^n-2^n)/2
L2_D
n-gon
Q: # diagonals?
Q: # intersections by diagonals?
Q: # segments coming from the diagonals?
