How do you evaluate #-3- 9+ 6+ 12- 5#?

1 Answer
EZ as pi
May 9, 2017

#1#

Explanation:

Calculations like these are often computed from left to right, but they can be done in any order because addition and subtraction are different forms of the same operations. One is the inverse of the other, they are equally strong.

HOWEVER!!
Students often get these wrong because they apply BODMAS, PEDMAS etc, without realising that a sign applies ONLY to the number on its right.

By trying to ADD first, (according to the rule) they incorrectly add as follows:

#-3-color(lime)(9+6+12)-5larr# there is a #+# sign between numbers

#-3-(color(lime)(9+6+12))-5# to get #-3-color(lime)(27)-5#

And then SUBTRACT as #-3-(color(magenta)(27-5))# to get #-3-color(magenta)(22)#

Leading to an incorrect answer of #-25#

My advice is that you can change the order AS YOU LIKE, as long as each sign stays with the correct number.

Write the #color(blue)("adds")# at the beginning and the #color(red)("subtracts")# at the end and you should get a correct result every time.

#color(red)(-3-9 )color(blue)( + 6+12)color(red)(-5)#

#= color(blue)(+6+12)color(red)(-3-9-5)#

#= color(blue)(+18)color(red)(-17)#

#=1#

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