△A refers to Class A uncertainty, which is used to evaluate system error. Usually, the tolerance of the instrument is directly taken as △ A, and △ B refers to the random error of measurement. When calculating △ B, we should first consider whether the measurement is a single measurement or multiple measurements, and then consider whether it is a direct measurement or an indirect measurement. For direct measurement, calculate △A△B respectively, and then synthesize the uncertainty. For indirect measurement, the combined uncertainty of each directly measured physical quantity should be calculated first, and then the uncertainty of indirectly measured physical quantity should be calculated according to the error transfer formula. The experiment of measuring density is indirect measurement. You should first calculate the combined uncertainty of each directly measured physical quantity, and then calculate the uncertainty of density.