# The Difference Between the Exterior Angles of Two Regular Polygons, Having the Sides Equal to (N – 1) and (N + 1) is 9°. Find the Value of N. - Mathematics

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The difference between the exterior angles of two regular polygons, having the sides equal to (n – 1) and (n + 1) is 9°. Find the value of n.

#### Solution

We know that sum of exterior angles of a polynomial is 360°

(i) If sides of a regular polygon = n – 1

Then each angle = 360^circ/("n" - 1)

and if sides are n + 1, then

each angle = 360^circ/("n" + 1)

According to the condition,

360^circ/("n" - 1) - 360^circ/("n" + 1)=9

=> 360 [1/("x" - 1) - 1/("x" + 1)] = 9

=> 360 [("n" + 1 - "n" + 1)/("n" - 1)("n" + 1)] = 9

=> (2 xx 360)/("n"^2 - 1) = 9

=> "n"^2 - 1 = (2 xx 360)/9 = 80

=> n^2 - 1 = 80

=> n^2 = 1 - 80 = 0

⇒ n2 - 81 = 0

⇒ (n)2 - (9)2 = 0

⇒ (n + 9)(n - 9) = 0

Either n + 9 = 0. then n = -9 which is not possible being negative,

or n - 9 = 0, then n = 9

∴ n = 9

∴ No. of. sides of a regular polygon = 9

Concept: Regular Polynomial
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#### APPEARS IN

Selina Concise Mathematics Class 8 ICSE
Chapter 16 Understanding Shapes
Exercise 16 (B) | Q 15 | Page 184