Why is the square root of a negative number undefined?

The square root of a negative number is undefined because there is no real number which, when squared, is negative.

A negative number is not a real number which, squared, is negative. 
You can't get square a number and get a negative. If you square a negative number, it'll become positive, therefore you can't square root a negative number because it doesn't exist. 
Hope this helps!

Add the cents that add the left overs to get the first number