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Question

Two angles of a polygon are right angles and every other angle is 120 degree. Find the number of sides of the polygon.

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Solution

Property of Polygon -
"Sum of all interior angles of n sided polygon is equals to 180 cross times left parenthesis n minus 2 right parenthesis"

It is given that two angles are 90 degree and rest are 120 degree
Thus sum of all angles from given data =
left parenthesis 90 cross times 2 right parenthesis space plus space 120 cross times left parenthesis n minus 2 right parenthesis equals 180 space plus 120 n space minus 240 equals 120 n space minus 60

From the above property of polygon,
Sum of all interior angles = 180 cross times(n-2) = 180n -360

By equating both,
120n -60 =180n -360
300 = 60n
n =5

Therefore number of side of polygon is 5

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