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    A bag is useful and the word "bag" is useful, too. Here is an interesting phrase, "to let the cat out of the bag". In meaning it is the same as "to tell the secret". And there is an old interesting story about it. Long ago, when farmers wanted to sell chickens at the market, they would usually throw the chickens in cloth bags. But cats were cheaper than chickens at that time, so many farmers would put cats into the cloth bags but not chickens.

    One day, a woman asked a man for a chicken. The man gave her a cloth bag in which there was a cat. The woman begged to see the chicken. When the man opened the cloth bag, a big black cat ran out. Not a chicken! The man's secret was out and everyone knew it.

    Now when we say someone gives away a secret, we say "he lets the cat out of the bag". And that is the story where the interesting phrase came from.

你想知道今天天气怎样，可以说：          

如果方程 与方程 有一个公共根是3，求 的值，并分别求出两个方程的另一个根．