A Triangle Problem!

Geometry Level 3

If a 3 + b 3 + c 3 a + b + c = c 2 \dfrac{a^3 + b^3 + c^3}{a + b + c} = c^2 and A A B C a b = ω λ \dfrac{A_{\triangle{ABC}}}{ab} = \dfrac{\sqrt{\omega}}{\lambda} , where ω \omega and λ \lambda are coprime positive integers,

find ω + λ \omega + \lambda .


The answer is 7.

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3 solutions

Hongqi Wang
Apr 5, 2021

a 3 + b 3 + c 3 a + b + c = c 2 a 3 + b 3 + c 3 = ( a + b + c ) c 2 c 2 = a 3 + b 3 a + b = a 2 a b + b 2 c 2 = a 2 + b 2 2 a b cos C cos C = 1 2 C = π 3 S = 1 2 a b sin C S a b = 1 2 sin C = 1 2 sin π 3 = 3 4 ω = 3 , λ = 4 , ω + λ = 7 \dfrac {a^3+b^3+c^3}{a+b+c} = c^2 \\ a^3+b^3+c^3 = (a+b+c)c^2 \\ c^2 = \dfrac {a^3+b^3}{a+b} = a^2 - ab + b^2 \\ c^2 = a^2 + b^2 - 2ab\cos C \\ \therefore \cos C = \dfrac 12 \implies C = \dfrac {\pi}3 \\ S_{\triangle} = \dfrac 12 ab\sin C \\ \dfrac {S_{\triangle}}{ab} = \dfrac 12 \sin C = \dfrac 12 \sin {\dfrac {\pi}3} = \dfrac {\sqrt 3}4 \\ \implies \omega = 3, λ = 4, \omega + λ = \boxed {7}

David Vreken
Apr 4, 2021

a 3 + b 3 + c 3 a + b + c = c 2 \cfrac{a^3 + b^3 + c^3}{a + b + c} = c^2 rearranges to c 2 = a 2 a b + b 2 c^2 = a^2 - ab + b^2 .

By a variation of Heron's formula , A A B C = 1 4 4 a 2 b 2 ( a 2 + b 2 c 2 ) 2 A_{\triangle ABC} = \cfrac{1}{4}\sqrt{4a^2b^2 - (a^2 + b^2 - c^2)^2} , which after substituting c 2 = a 2 a b + b 2 c^2 = a^2 - ab + b^2 simplifies to A A B C = 3 4 a b A_{\triangle ABC} = \cfrac{\sqrt{3}}{4}ab .

Therefore, A A B C a b = 3 4 a b a b = 3 4 \cfrac{A_{\triangle ABC}}{ab} = \cfrac{\frac{\sqrt{3}}{4}ab}{ab} = \cfrac{\sqrt{3}}{4} , so ω = 3 \omega = 3 , λ = 4 \lambda = 4 , and ω + λ = 3 + 4 = 7 \omega + \lambda = 3 + 4 = \boxed{7} .

Rocco Dalto
Apr 4, 2021

Let a 3 + b 3 + c 3 a + b + c = c 2 \dfrac{a^3 + b^3 + c^3}{a + b + c} = c^2

Using the law of cosines c 2 = a 2 + b 2 2 a b cos ( α ) \implies c^2 = a^2 + b^2 - 2ab\cos(\alpha) \implies

a 3 + b 3 + c 3 = c 2 ( a + b + c ) = ( a + b + c ) ( a 2 + b 2 2 a b cos ( α ) ) = a^3 + b^3 + c^3 = c^2(a + b + c) = (a + b + c)(a^2 + b^2 - 2ab\cos(\alpha)) =

a 3 + b 3 + a b 2 + b a 2 + c a 2 + c b 2 2 ( a 2 b + a b 2 + a b c ) cos ( α ) ) a^3 + b^3 + ab^2 + ba^2 + ca^2 + cb^2 - 2(a^2b + ab^2 + abc)\cos(\alpha)) \implies

c 3 c ( a 2 + b 2 ) = a b 2 + b a 2 2 ( a 2 b + a b 2 + a b c ) cos ( α ) ) c^3 - c(a^2 + b^2) = ab^2 + ba^2 - 2(a^2b + ab^2 + abc)\cos(\alpha)) \implies

c ( c 2 ( a 2 + b 2 ) ) = a b 2 + b a 2 2 ( a 2 b + a b 2 + a b c ) cos ( α ) c(c^2 - (a^2 + b^2)) = ab^2 + ba^2 - 2(a^2b + ab^2 + abc)\cos(\alpha) \implies

c ( 2 a b cos ( α ) ) = 2 a b c cos ( α ) = a b 2 + b a 2 2 ( a 2 b + a b 2 + a b c ) cos ( α ) c(-2ab\cos(\alpha)) = -2abc\cos(\alpha) = ab^2 + ba^2 - 2(a^2b + ab^2 + abc)\cos(\alpha) \implies

a b 2 + b a 2 2 ( a 2 b + a b 2 ) cos ( α ) = 0 ( a b 2 + b a 2 ) ( 1 2 cos ( α ) ) = 0 ab^2 + ba^2 - 2(a^2b + ab^2)\cos(\alpha) = 0 \implies (ab^2 + ba^2)(1 - 2\cos(\alpha)) = 0 \implies

a b ( a + b ) ( 1 2 cos ( α ) ) = 0 ab(a + b)(1 - 2\cos(\alpha)) = 0 and a > 0 a > 0 and b > 0 1 2 cos ( α ) = 0 b > 0 \implies 1 - 2\cos(\alpha) = 0 \implies

cos ( α ) = 1 2 α = 6 0 \cos(\alpha) = \dfrac{1}{2} \implies \alpha = 60^{\circ} \implies

A A B C = 1 2 a b sin ( 6 0 ) = 3 4 a b A A B C a b = 3 4 = ω λ A_{\triangle{ABC}} = \dfrac{1}{2}ab\sin(60^{\circ}) = \dfrac{\sqrt{3}}{4}ab \implies \dfrac{A_{\triangle{ABC}}}{ab} = \dfrac{\sqrt{3}}{4} = \dfrac{\sqrt{\omega}}{\lambda}

ω + λ = 7 \implies \omega + \lambda = \boxed{7} .

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