MAths

sum of interior angle; (n - 2)180
sum of exterior angle = 360
so, (n-2)180/n - 360/n = 100
180n - 360 -360 = 100n
80n = 720
n = 9
Therefore, the no. of sides of this polygon is 9.

2007-11-08 19:33:17 補充：
I forget to say "Let n be the no. of sides of this polygon." at the beginning.SORRY~~~

Let X be the number of sides of this polygon.
Then the total sum of all the interor angles = 180 (X- 2) degrees
Each interior angle = 180 (X -2)/X degrees
As the sum of external angles is 360 degrees for any n-side polygon, each external angle is 360/X degrees
180 (X -2)/X - 360/X = 100
180(X-2) - 360 = 100X
180X - 360 - 360 = 100X
180X - 720 = 100X
80X = 720
X = 9
Answer: The number of sides of this polygon is 9.