How do you subtract #-3.1 - 21.9#?

1 Answer
Alan P.
Jun 27, 2018

Mostly likely: #-3.1=-21.9=color(blue)(-25.0)#
...but see below

Explanation:

Normal evaluation is from left to right;
so #-3.1-21.9#
would commonly be evaluated as
#color(white)("XXX")(-3.1)-(21.9)#
#color(white)("XXX")=color(white)("xx")-3.1color(white)("xxx")3.1# units to the left of zero on the number line
#color(white)("XXX"=)-ul(color(white)("x")21.9color(white)("xxx")#move #21.9# units further to the left
#color(white)("XXX")=color(white)("x")-25.0color(white)("xx")# now a total of #25# units to the left of zero.

#ul("However")#
Sometimes evaluation is done right to left:
So #-3.1-21.9#
would be evaluated as #-(3.1-21.9)=-(-17.2)=+17.2#

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