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I don't think they are wrong. I have ##x:=AB## and thus ##AC=3x## opposite of ##\beta = \angle (ABC) = 60°-\theta ## and ##x=AB## opposite of ##\gamma = \angle (BCA) =\theta##.
You can get rid of ##x## because you're not interested in its value. ##\sin \theta## is what we're looking for.
You can get rid of ##x## because you're not interested in its value. ##\sin \theta## is what we're looking for.