If you had one negative number in it, then the product would be a negative number and, if you took a root of that, then you might get an imaginary number.

如果这当中有一个负数,乘积就会变成负的,如果你对一个负数开根号,你就会得到一个虚数

耶鲁公开课 - 金融市场课程节选

Remember, you've got to keep track of the sign of acceleration, so if you're picking up speed towards the ground, a is negative, so it will be g plus a, but a is a negative number.

记住,一定要注意加速度的符号,所以当你加速下降的时候,a 是负的,还是 g 加上 a,但是 a 是负数

耶鲁公开课 - 基础物理课程节选