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https://chinese.stackexchange.com/questions/755/nu…
Number two in chinese: 二 vs 两
二项式 (binomial), 二氧化碳 (CO 2),三氧化二铝 (Al 2 O 3), 二次曲线 (curve of the second degree), 二元方程 (two element equation), 二阶微分方程 (two order differential equation). In numbers, when two is used at the digit fewer than hundred, only 二 is used. In other cases, 两 could also be used (they are ...