The sum of the internal angles of an n-sided polygon is given by 180o • (n – 2)
So for an octagon (8 sides) the sum of its internal angles equals 180o • (8 – 2)
Doing the arithmetic, this sum is 1,080o
Since it is a regular octagon, all internal angles are equal, so each one is 135o
x is a supplementary angle of one of those 135o angles, so it equals 180o – 135o
Thus, x = 45o
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