Let p be the number of all triangles that can be formed by joining the vertices of a regular polygon P of n sides and q be the number of all quadrilaterals that can be formed by joining the vertices of P. If p + q = 126, then the eccentricity of the ellipse x216 + y2n = 1 is :
- 1 2
- 12
- 74
- 34
Answer
(1) 1 2
Solution
No three vertices of a regular polygon are collinear, so
p = nC3 and q = nC4
p + q = nC3 + nC4 = n+1C4 = 126
9C4 = 126, n + 1 = 9 n = 8
Ellipse: x216 + y28 = 1, with a2 = 16, b2 = 8
e = 1 - b2a2 = 1 - 816 = 1 2
Answer: (1)