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No integer roots (Posted on 2017-10-22) Difficulty: 3 of 5
Without solving the equation
15x^2-7y^2=9
explain why it has no integer roots.

No Solution Yet Submitted by Ady TZIDON    
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No Subject | Comment 1 of 4
The last digit of 15x^2 is 0 or 5.

The last digit of 7y^2 is (0,7,8,3,2,5,2,3,8,7) according as the last digit of y is (0,1,2,3,4,5,6,7,8,9).

In none of the cases is it possible for the last digit of the difference to equal 9. 

  Posted by xdog on 2017-10-22 17:42:51
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