It's all Hexagons

Level pending

If h = B D h^{*} = BD is the height of A B C \triangle{ABC} and a side of the inscribed regular hexagon c = 2 3 h c = 2\sqrt{3} h^{*} and the area A A of the larger hexagon is A = h 4 + 432 A = {h^{*}}^{4} + 432 , find the sum of the areas of the red triangles.


The answer is 216.

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1 solution

Rocco Dalto
Jul 26, 2018

Using the law of sines on A B C \triangle{ABC} we obtain:

4 h = a sin ( α ) sin ( α ) = a 4 h h = a b 4 h b = 4 h 2 a 4h^{*} = \dfrac{a}{\sin(\alpha)} \implies \sin(\alpha) = \dfrac{a}{4h^{*}} \implies h^{*} = \dfrac{ab}{4h^{*}} \implies b = \dfrac{4{h^{*}}^{2}}{a} .

Using the law of cosines a 2 + a b + b 2 = 12 h 2 a 2 + 4 h 2 + 16 h 4 a 2 = 12 h 2 a 4 8 h 2 a 2 + 16 h 4 = 0 \implies a^2 + ab + b^2 = 12{h^{*}}^2 \implies a^2 + 4{h^{*}}^2 + \dfrac{16{h^{*}}^4}{a^2} = 12{h^{*}}^2 \implies a^4 - 8{h^{*}}^2 a^2 + 16 {h^{*}}^4 = 0 \implies

( a 2 4 h 2 ) 2 = 0 a = 2 h b = 2 h a + b = 4 h (a^2 - 4{h^{*}}^2)^2 = 0 \implies a = 2h^{*} \implies b = 2h^{*} \implies a + b = 4h^{*} \implies

the area of the larger hexagon A = 24 3 h 2 = h 4 + 432 h 4 24 3 h 2 + ( 12 3 ) 2 = 0 ( h 2 12 3 ) 2 = 0 h 2 = 12 3 A = 24\sqrt{3}{h^{*}}^2 = {h^{*}}^{4} + 432 \implies {h^{*}}^{4} - 24\sqrt{3}{h^{*}}^2 + (12\sqrt{3})^2 = 0 \implies ({h^{*}}^2 - 12\sqrt{3})^2 = 0 \implies {h^{*}}^2 = 12\sqrt{3} h = 2 27 4 \implies h = 2\sqrt[\scriptstyle 4]{27}

\implies The sum of the areas of the red triangles j = 1 6 ( A r e d ) j = 6 ( 1 2 ) ( 2 3 h 2 ) = 6 3 h 2 = 6 3 ( 12 3 ) = 6 12 3 = 216 \sum_{j = 1}^{6} (A_{red})_{j} = 6(\dfrac{1}{2})(2\sqrt{3}{h^{*}}^2) = 6\sqrt{3}{h^{*}}^2 = 6\sqrt{3}(12\sqrt{3}) = 6 * 12 * 3 = \boxed{216} .

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