addition and subtraction

When we have two whole numbers:

- if both have the same sign, we add their absolute values and put the common sign:        

3 + 4 = 7                      - 3 - 4 = - 7

- If both have different signs, we subtract the  bigger absolute value minus the lower absolute value and put the sign of the number which has a bigger absolute value:
3 – 7 = - 4                     11 – 7 = 4

When we have more than two whole numbers:

- we add the positive numbers
- separately, we add the absolute values of the negative numbers with the negative sign
- at the end, we add these whole numbers with different sign:

NOTE:  If there is a negative sign before a bracket, we change the sign of the result of the bracket (or the sign of all the numbers inside the bracket) but we don´t change it if the sign before the bracket is positive:

 

Exercise.- Calculate:

a) -3 - 7 =

b) 3 - 11 =

c) 7- (-5) + (4 - 5)- (3 - 8 - 4) =

d) 3 + [4 - (5 + 8)] - 5 - (-2) + (-1) =

e) (16 - 9) - (4 - 5) - (-3) + 8 =  

 

 

 

Solutions:a) -10; b) -8; c) 20; d) -10; e) 19