For how many triples of integers between -10 and 10 inclusive do there exist reals that satisfy
Solution
If none are of are zero, then there are ways, since must be positive. Indeed, . So an even number of them are negative, and the ways to choose an even number of 3 variables to be negative is 4 ways. If one of is 0 , then one of is zero at least. So at least two of must be 0 . If all 3 are zero, this gives 1 more solution. If exactly 2 are negative, then this gives more solutions. This comes from choosing one of to be nonzero, and choosing its value in 20 ways. Our final answer is .
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