Odpowiedź:
an = n² + n - 20
a(n - 1) = (n - 1)²+ (n - 1) - 20 = n² - 2n + 1 + n - 1 - 20 = n² - n - 20
a(n + 1 )= (n+ 1)² + n + 1 - 20 = n² + 2n + 1 + n - 19 = n² + 3n - 18
a(2n + 1) = (2n + 1)² + 2n + 1 - 20 = 4n² + 4n + 1 + 2n - 19 = 4n² + 6n - 18
