In a fraction, the denominator is the number under the line. In 3/4, four is the denominator: it tells you how many equal parts the whole was cut into. The number on top is the numerator, which tells you how many of those parts you have. So 3/4 means “three of four equal parts.” You need a common denominator when you add or compare fractions — a shared bottom number so the parts are the same size.

In a fraction在一個分數裡，the denominatordenominatoris the number under the line是分數線下面的那個數字。In 3/4在3/4裡，four is the denominator4就是denominator：it tells you how many equal parts the whole was cut into它告訴你整體被切成了幾等分。The number on top上面的那個數字is the numerator叫做numerator，which tells you how many of those parts you have它告訴你你拿到了幾份。So所以3/4 means "three of four equal parts."3/4的意思就是「四等分裡的三份」。You need a common denominator你需要一個共同的分母when you add or compare fractions當你要把分數相加或比較的時候——a shared bottom number就是一個共用的分母數字so the parts are the same size讓每一份的大小一致。

Though actually, people also stretch this in ordinary talk: “the common denominator” can mean the one thing several people or problems share. That everyday use grew out of the school meaning, and it is very common. The word itself is maths-class English first, then metaphor.

Though actually不過其實，people also stretch this in ordinary talk人們在日常說話裡也會延伸這個用法："the common denominator"the common denominatorcan mean the one thing several people or problems share可以指好幾個人或好幾個問題共同的那一點。That everyday use這種日常用法grew out of the school meaning是從學校教的意思延伸出來的，and it is very common而且用得很普遍。The word itself這個字本身is maths-class English first一開始是數學課的英文，then metaphor後來才變成比喻用法。