A fifth in 12-TET is defined as seven semitones. Instead of multiplying your frequency by 3/2, you multiply by the 12th root of two seven times, which is about 1.498. That’s close, but not quite on the nose. Major thirds are worse in 12-TET. Instead of multiplying your frequency by 5/4, you multiply by the 12th root of two four times. That gives you 1.25992, which is not very close to 5/4 at all. Nevertheless, we as a civilization have collectively decided that we should just suck it up and live with everything sounding a little wrong. There are plenty of good reasons to!
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