Balmer, a Swiss middle school teacher, found the rule from the spectral data 5 / 9, 12 / 16, 21 / 25, 32 / 36. Please write the number n according to the rule

Balmer, a Swiss middle school teacher, found the rule from the spectral data 5 / 9, 12 / 16, 21 / 25, 32 / 36. Please write the number n according to the rule

The denominator is the square
The numerator is four times smaller than the denominator
So it's [(n + 2) & # 178; - 4] / (n + 2) & # 178;
=(n²+4n)/(n+2)²