Answer:
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Y=10°
If we're solving for the ninety degree angle, which I seem to doubt, then if 3x=90 then 2x=60 giving a missing total of 30. 30/3 equals 10
Answer:
a) Suppose that F is ordered in ascending order: 
. Then, the complement of F can be written as
which is the union of a finite number of open intervals, then 
is an open set. Thus, F is a closed subset of the real numbers.
b) Take an arbitrary element of F, let us say 
. Now, choose a real number 
such that
there are not other element of F, because is less that the minimum distance between and its neighbors.
In case that 
we only consider 
, and if 
we only consider 
.
Then, all points of F are isolated.
Step-by-step explanation:
