Question

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 $ \frac{x^2}{16} + \frac{y^2}{n} = 1 $ is:

• A. \( \frac{3}{4} \)
• B. \( \frac{1}{2} \)
• C. \( \frac{\sqrt{7}}{4} \)
• D. \( \frac{1}{\sqrt{2}} \)

Hint:

When calculating the eccentricity of an ellipse, first find the values of \( a^2 \) and \( b^2 \) from the equation, then use the formula \( e = \sqrt{1 - \frac{b^2}{a^2}} \).

Answer 1

The Correct Option is D

We are given that \( p + q = 126 \), where \( p \) is the number of triangles and \( q \) is the number of quadrilaterals formed by the vertices of a regular polygon with \( n \) sides.

Step 1: Calculate \( p \) and \( q \).

The number of triangles formed by selecting 3 vertices from \( n \) vertices is given by: \[ p = \binom{n}{3} = \frac{n(n-1)(n-2)}{6}. \] The number of quadrilaterals formed by selecting 4 vertices from \( n \) vertices is given by: \[ q = \binom{n}{4} = \frac{n(n-1)(n-2)(n-3)}{24}. \] Given that \( p + q = 126 \), we can substitute the expressions for \( p \) and \( q \) and solve for \( n \). \[ \frac{n(n-1)(n-2)}{6} + \frac{n(n-1)(n-2)(n-3)}{24} = 126. \] Multiply through by 24 to eliminate fractions: \[ 4n(n-1)(n-2) + n(n-1)(n-2)(n-3) = 3024. \] Now, solve this equation for \( n \).
Step 2: Solve for \( n \).
Solving the above equation, we find that \( n = 8 \).

Step 3: Find the eccentricity.

The equation of the ellipse is given by: \[ \frac{x^2}{16} + \frac{y^2}{n} = 1. \] Substitute \( n = 8 \) into this equation: \[ \frac{x^2}{16} + \frac{y^2}{8} = 1. \] The standard form of an ellipse is: \[ \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1, \] where \( a^2 = 16 \) and \( b^2 = 8 \). The eccentricity \( e \) of the ellipse is given by: \[ e = \sqrt{1 - \frac{b^2}{a^2}}. \] Substitute \( a^2 = 16 \) and \( b^2 = 8 \): \[ e = \sqrt{1 - \frac{8}{16}} = \sqrt{1 - \frac{1}{2}} = \sqrt{\frac{1}{2}} = \frac{1}{\sqrt{2}}. \]