When students use calculator to calculate the average of 30 data, they input one of the data 108 as 18, and the difference between the calculated average and the actual average is much

When students use calculator to calculate the average of 30 data, they input one of the data 108 as 18, and the difference between the calculated average and the actual average is much

Less 108 / 30-18 / 30 = 3

A student used a calculator to calculate the average of 30 numbers, and mistakenly pressed 108 into 18, so as to find out the difference between the average and the actual average

Let the sum of the other 29 numbers be a
Then:
The actual average is
(a+108)/30.
The average is
(a+18)/30.
The difference between the calculated average and the actual average is
((a+18)/30)-((a+108)/30)
=((a+18)-(a+108))/30
=(a+18-a+108)/30
=-90/30
=-3

If the sum of two rational numbers is negative, then the two numbers must ()
A. All negative numbers B. at least one negative number C. one is 0d. The absolute values are equal

A. It is uncertain, for example: - 5 + 2 = - 3; B, correct; C, uncertain, for example: - 5 + 2 = - 3; D, uncertain, for example: - 8 + 8 = 0

If the sum of two rational numbers is negative, then these two numbers must be negative. Is that right? If not, please give an example?

This is not true. For example, the sum of 8 + (- 10) = - 2 is - 2, which is negative, while the addend one positive and one negative is not all negative

If a is positive, then - a must be negative, right?

Exactly

The difference between two negative numbers must be a negative number,

For example - 1 - 6 = - 1 + 6 = 5

Positive and negative numbers are opposite to each other,

Not rigorous!
It should be said that:
A positive number and its corresponding negative number are opposite to each other

-|-The number a | represents may be a choice (positive number) (negative number) (positive number or 0) (negative number or 0). Which is it?

Negative number or 0
Because | - a | must not be negative, positive or zero
So - | - a | - is negative or zero

Using a to represent a number, how to represent that a is a positive number, a is a negative number, and a is 0

A is a positive number: a > 0
A is a negative number: a

The absolute value of negative number and zero is his opposite number, right?

The opposite of zero is zero (think about it carefully)