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The answer to the fourth issue of the eighth grade mathematics newspaper in Jiangxi special issue is known as the sum of the inner and outer angles of a polygon.
N polygons have n vertices.

At each vertex, internal angle+external angle = 180.

The sum of polygon outer angles = 360 = 2 * 180.

Sum of internal angles = n *180-360 = (n-2) *180 = 7 *180.

So n-2=7

n=9

enneagon