1990 AIME Problems/Problem 10
Problem
The sets 
and 
are both sets of complex roots of unity. The set 
is also a set of complex roots of unity. How many distinct elements are in 
?
Solution
Solution 1
The least common multiple of 
and 
is 
, so define 
. We can write the numbers of set 
as 
and of set 
as 
. 
can yield at most different values. All solutions for 
will be in the form of 
.
and 
are relatively prime, and by the Chicken McNugget Theorem, for two relatively prime integers 
, the largest number that cannot be expressed as the sum of multiples of is 
. For 
, this is 
; however, we can easily see that the numbers 
to 
can be written in terms of . Since the exponents are of roots of unities, they reduce 
, so all numbers in the range are covered. Thus the answer is 
.
Solution 2
The 18 and 48th roots of 
can be found by De Moivre's Theorem. They are 
and 
respectively, where 
and 
and 
are integers from 
to 
and to 
, respectively.
. Since the trigonometric functions are periodic every 
, there are at most 
distinct elements in 
. As above, all of these will work.
Solution 3
The values in polar form will be (1, 20x) and (1, 7.5x). Multiplying these gives (1, 27.5x). Then, we get 27.5, 55, 82.5, 110, ... up to 3960 (lcm(55,360)) => 3960*2/55=144.
See also
| 1990 AIME (Problems • Answer Key • Resources) | ||
| Preceded by Problem 9 |
Followed by Problem 11 | |
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