Ordered Triples Counting
For how many ordered triples $ (x,y,z) $ of nonnegative integers less than $ 20 $ are there exactly two distinct elements in the set $ \{i^x, (1+i)^y, z\} $, where $ i^2 = -1 $?
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
We divide into cases.
Case 1: $ i^x = (1 + i)^y \neq z $.
Note that $ |i^x| = |i|^x = 1 $ and $ |(1 + i)^y| = |1 + i|^y = (\sqrt{2})^y, $ so we must have $ y = 0 $. Then $ i^x = 1 $ only when $ x $ is a multiple of 4. There are 5 possible values of $ x $ (0, 4, 8, 12, 16), and 19 possible values of $ z, $ so there are $ 5 \cdot 19 = 95 $ triples in this case.
Case 2: $ i^x = z \neq (1 + i)^y $.
The only way that $ i^x $ can be a nonnegative integer is if it is equal to 1, which in turn means that $ x $ is a multiple of 4. As in case 1, $ |(1 + i)^y| = (\sqrt{2})^y, $ so $ (1 + i)^y \neq 1 $ is satisfied as long as $ y \neq 0 $. This gives us 5 possible values of $ x, $ and 19 possible values of $ y, $ so there are $ 5 \cdot 19 = 95 $ triples in this case.
Case 3: $ (1 + i)^y = z \neq i^x $.
Note that $ (1 + i)^2 = 2i, $ and we must raise $ 2i $ to a fourth power to get a nonnegative integer. Hence, $ (1 + i)^y $ is a nonnegative integer only when $ y $ is a multiple if 8. Furthermore, $ (1 + i)^8 = (2i)^4 = 16, $ and $ (1 + i)^{16} = 16^2 = 256, $ so the only possible values of $ y $ are 0 and 8.
For $ y = 0, $ $ z = 1, $ and then $ x $ cannot be a multiple of 4. This gives us $ 20 - 5 = 15 $ triples.
For $ y = 8, $ $ z = 16, $ and $ x $ can take on any value. This gives us 20 triples, so there are $ 15 + 20 = 35 $ triples in this case.
Therefore, there are a total of $ 95 + 95 + 35 = \boxed{225} $ triples.