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

The law of molecules: 5 12 21 32 45 60 77, followed by the increasing odd number 5 5 + 7 12 + 9 21 + 11 32 + 13 45 + 15 60 + 17 = 77
The law of denominator: 9 16 25 36 49 64 81 is the square of the larger integer 3 ^ 2 4 ^ 2 5 ^ 2 6 ^ 2 7 ^ 2 8 ^ 2 9 ^ 2 in turn
So the answer is 77 / 81