A Special Angle 2

Level 2

In quadrilateral A B C D ABCD , M 1 , M 2 , M 3 M_{1}, M_{2}, M_{3} and M 4 M_{4} are midpoints of A B , C D , A D \overline{AB}, \overline{CD}, \overline{AD} and B C \overline{BC} respectively, A B C A D C \triangle{ABC} \cong \triangle{ADC} and P P is the centroid of quadrilateral A B C D ABCD obtained by finding the intersection of the bimedians.

In A B C , A C = r 1 a , A B = r 2 a \triangle{ABC}, \overline{AC} = r_{1}a, \overline{AB} = r_{2}a and B C = r 3 a \overline{BC} = r_{3}a .

Let Q P = h QP = h be the height of the pyramid.

Find the value of a a and h h that minimizes the triangular face Q D C QDC .

Find m Q M 2 P m\angle{QM_{2}P} and express the result to seven decimal places.

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The answer is 54.7356103.

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1 solution

Rocco Dalto
Oct 27, 2018

A B C A D C A A B C D = r 1 r 2 sin ( ω ) a 2 \triangle{ABC} \cong \triangle{ADC} \implies A_{ABCD} = r_{1}r_{2}\sin(\omega)a^2

The bi-medians of a quadrilateral bisect each other and the diagonals of a quadrilateral are perpendicular \implies the bi-medians are congruent \implies the centroid P P is the midpoint of M 1 M 2 P ( 2 r 2 cos ( ω ) + r 1 4 a , 0 ) \overline{M_{1}}\overline{M_{2}} \implies P(\dfrac{2r_{2}\cos(\omega) + r_{1}}{4}a,0) .

P M 2 = a 2 r 1 2 + 4 r 2 2 sin 2 ( ω ) A = A Q D C = PM_{2} = \dfrac{a}{2}\sqrt{r_{1}^2 + 4r_{2}^2\sin^2(\omega)} \implies A = A_{\triangle{QDC}} = a r 3 4 4 h 2 + ( r 1 2 + 4 r 2 2 sin 2 ( ω ) ) a 2 \dfrac{ar_{3}}{4}\sqrt{4h^2 + (r_{1}^2 + 4r_{2}^2\sin^2(\omega))a^2}

The volume of the pyramid V = 1 3 ( r 1 r 2 sin ( ω ) ) a 2 h = K h = 3 k r 1 r 2 sin ( ω ) a 2 V = \dfrac{1}{3}(r_{1}r_{2}\sin(\omega))a^2h = K \implies h = \dfrac{3k}{r_{1}r_{2}\sin(\omega)a^2}

Let j = ( r 1 2 r 2 2 sin 2 ( ω ) ) ( r 1 2 + 4 r 2 2 sin 2 ( ω ) ) P M 2 = j a 2 r 1 r 2 sin ( ω ) j = (r_{1}^2 r_{2}^2\sin^2(\omega))(r_{1}^2 + 4r_{2}^2\sin^2(\omega)) \implies \overline{PM_{2}} = \dfrac{\sqrt{j}a}{2r_{1}r_{2}\sin(\omega)} and A ( a ) = r 3 36 k 2 + j a 6 4 r 1 r 2 sin ( ω ) a A(a) = r_{3}\dfrac{\sqrt{36k^2 + ja^6}}{4r_{1}r_{2}\sin(\omega)a} \implies d A d a = r 3 j a 6 18 k 2 ( 4 r 1 r 2 sin ( ω ) a 2 36 k 2 + j a 6 a = ( 3 2 k j ) 1 3 \dfrac{dA}{da} = r_{3}\dfrac{ja^6 - 18k^2}{(4r_{1}r_{2}\sin(\omega)a^2\sqrt{36k^2 + ja^6}} \implies a = (\dfrac{3\sqrt{2}k}{\sqrt{j}})^{\frac{1}{3}} h = 3 k r 1 r 2 sin ( ω ) ( j 3 2 k ) 2 3 \implies h = \dfrac{3k}{r_{1}r_{2}\sin(\omega)}(\dfrac{\sqrt{j}}{3\sqrt{2}k})^{\frac{2}{3}}

h a = j 2 r 1 r 2 sin ( ω ) tan ( θ ) = ( 2 r 1 r 2 sin ( ω ) j ( h a ) = 2 θ 54.7356103 \dfrac{h}{a} = \dfrac{\sqrt{j}}{\sqrt{2}r_{1}r_{2}\sin(\omega)} \implies \tan(\theta) = (\dfrac{2r_{1}r_{2}\sin(\omega)}{\sqrt{j}}(\dfrac{h}{a}) = \sqrt{2} \implies \theta \approx \boxed{54.7356103} .

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