She had known Max for some years now, but he was still pretty much an unknown quantity.

她现在认识马克斯也好几年了，但是他仍然是个大大的未知数。

Are an unknown quantity.

对于他们来说是一个未知的组合。

We are facing an unknown purpose unknown quantity.

们常常是对着一个未知数而前进。

To her the outside world was a totally unknown quantity.

对她来说，外部世界完全是个未知数。

Unknown Quantity: a Real and Imaginary History of algebra.

《隐蔽的数量：代数的真实和想象的历史》。