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jmc

algebra senior

Problem

For how many ordered triples of nonnegative integers less than are there exactly two distinct elements in the set , where ?
Solution
We divide into cases.

Case 1:

Note that and so we must have Then only when is a multiple of 4. There are 5 possible values of (0, 4, 8, 12, 16), and 19 possible values of so there are triples in this case.

Case 2:

The only way that can be a nonnegative integer is if it is equal to 1, which in turn means that is a multiple of 4. As in case 1, so is satisfied as long as This gives us 5 possible values of and 19 possible values of so there are triples in this case.

Case 3:

Note that and we must raise to a fourth power to get a nonnegative integer. Hence, is a nonnegative integer only when is a multiple if 8. Furthermore, and so the only possible values of are 0 and 8.

For and then cannot be a multiple of 4. This gives us triples.

For and can take on any value. This gives us 20 triples, so there are triples in this case.

Therefore, there are a total of triples.
Final answer
225