[tex] \frac{13}{x^{2} +y^{2} } [/tex] supraunitara⇒13>x²+y²
13>x²+y² , x² patrat perfect, y² patrat perfect
suma de patrate mai mica decat 13 poate fi alcatuita de perechile {1, 2} si {1 si 3}
[tex] \frac{13}{x^{2} +y^{2} } = \frac{13}{1^{2} +2^{2} } = \frac{13}{1 +4} = \frac{13}{5} [/tex] - supraunitara
1²+2²=1+4=5
1²+3²=1+9=10
[tex]\frac{13}{x^{2} +y^{2} } = \frac{13}{1^{2} +3^{2} } = \frac{13}{1 +9} = \frac{13}{10}[/tex] - supraunitara
