7⁸ - 3⁸ = ( 7⁴)² - ( 3⁴)² = ( 7⁴ - 3⁴)·( 7⁴ + 3⁴ ) = ( 7² - 3²)·(7² + 3²)·(7⁴ +3⁴) =
= ( 49 - 9 )· ( 7² + 3²)·(7⁴ + 3⁴)
= 40· ( 7² + 3²)· (7⁴ + 3⁴ )
= 4 ·( 7² + 3²) ·(7⁴ + 3⁴ ) · 10 se divide cu 10
2 . a b ∈ N
2a + 3b =13
daca a = 2 atunci 2·2 + 3b = 13 ; 3b =13 -4 =9
si b = 3
daca a = 5 atunci 2·5 + 3b =13 ; 3b =13 -10 =3
si b = 1