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In the first article on the history of negative numbers, we were informed about the sign rules the Chinese used in order to create quantities with the chinese rods. We learnt that the red rods represent positive numbers and the black rods denote negative numbers.

The video on the use of chinese rods in creating quantities form the basis for how negative numbers work with positive numbers.

From his eleven statements, referring to negative numbers as ‘debt’ as opposed to property, Brahmagupta expounded on the rules for dealing with negative numbers.

His first statement, that ' A debt minus zero is a debt', implies that taking zero from a debt will neither change the debt nor make it a fortune.

However, the forth statement, 'A debt subtracted from zero is a fortune'. This implies that when you have a debt (negative number) and you subtract from zero (positive number), your debt is being cancelled (you will actually be adding) and that will be a fortune (positive).

Statement 9 states that, 'The product or quotient of two debts is one fortune', and this means that when you multiply two negative numbers you will get a positive number.

On the other hand the last two statements mean that multiplying a debt by a fortune (positive number) gives you a debt (negative number).

In conclusion the rules imply that multiplying or dividing same signs gives a positive answer. Opposite signs make the product or quotient negative.

Adding and subtracting both negative and positive numbers have different methods. Hence the video below explains them.

The following links are worksheets on examples of addition and subtraction with negative numbers for practise:

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