1. What is the biggest prime number?

No one has yet found the biggest prime number. It is probably an INFINITE set.

2. What is the square root of a negative number?

If you try to find the square root of -9, that is √(-9), a calculator will give you an error.
This is because there is no number that multiplies by itself to give -9.

So, at this stage, you cannot find the square root of a negative number. (Later, in the senior school you may learn about "imaginary numbers" which do help to deal with the square roots of negative numbers.)

3. What is the lowest prime number?

Many people think that ONE is the lowest prime number, but it does not count as a prime number. TWO is the first prime number and the only EVEN prime number.

4. Can you find the reciprocal of a negative number?

It does not matter whether the number is positive or negative, finding the reciprocal is done in the same way.

e.g. Reciprocal of -5 is 1/ -5 = − 0.2
Reciprocal of 5 is 1/ 5 = 0.2