# algebra

posted by unowen

15 can be written as the difference between two positive perfect squares:
15=4²-1²=16-1

Which of the following numbers cannot be written as the difference of two positive perfect squares?

16
17
18
19
20

1. PsyDAG

These are not differences.

17 = 4^2 + 1^2

16 = 2^2 * 2^2

20 = 4^2 + 2^2

18 = 3^2 + 3^2

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