Two normalized period/radius ledgers cite the same K source. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The first candidate cites the normalized period law
The period-law source is 1/2. The first candidate has 4 over 8.
K=21,a3T2=84=21
A second exact candidate matches the same source
The second candidate has 32 over 64, which also reduces to 1/2.