1. for every n 屬於Z^+, n>=14, prove that n can be written as a sum of
summands of 3's and/or 8's.
2. how many 20-digit quaternary (0,1,2,3)sequences are there, where:
a) there is at least one 2 and an odd number of 0's ?
3. Determine the last digit in (a) 3^55, and (b) 9^1989?
4. if G is a group of order n and a 屬於 G, prove that a^n = e.
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