Triangle Area Sum 2

Geometry Level 5

Find all possible areas of a triangle with 2 2 sides of lengths 11 11 and 12 12 and one 6 0 60^{\circ} angle.

If the sum of the areas of the 3 3 smallest triangles is a b , a\sqrt{b}, where a a and b b are integers and b b is square-free, enter your answer as a + b a+b .



The answer is 72.

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1 solution

Trevor Arashiro
Mar 15, 2015

Through this solution, I will refer to law of cosines a 2 = b 2 + c 2 2 b c cos ( A ) a^2=b^2+c^2-2bc\cos(A) as LOC and area of a triangle in terms of sine 1 2 a b sin C \frac{1}{2}ab\sin{C} as ATS. Also, lower case letters represent sides while uppercase letters represent angles.

Begin with the simple case of a = 11 , b = 12 C = 60 a=11,~b=12~C=60 .

ATS 1 2 ( 11 ) ( 12 ) ( 3 2 ) = 33 3 57.157 \text{ATS}\Longrightarrow \frac{1}{2}(11)(12)\left(\frac{\sqrt{3}}{2}\right)=33\sqrt{3}\approx 57.157

Now if a = 11 , b = 12 , A = 60 a=11,~b=12,~A=60

LOC a 2 = b 2 + c 2 2 b c cos ( A ) 121 = 144 + c 2 12 c 0 = c 2 12 c + 23 c = 6 ± 13 ATS 1 2 ( 6 ± 13 ) ( 12 ) ( 3 2 ) = 18 3 ± 3 39 49.9 , 12.4 \begin{aligned} \text{LOC}\longrightarrow a^2&=b^2+c^2-2bc\cos(A)\\ 121&=144+c^2-12c\\ 0&=c^2-12c+23\\ c&=6\pm \sqrt{13}\\ \text{ATS}& \Longrightarrow \frac{1}{2}(6\pm \sqrt{13})(12)\left(\frac{\sqrt{3}}{2}\right)\\ &=18\sqrt3\pm3\sqrt{39}\\ &\approx 49.9,~12.4 \end{aligned}

Now if a = 11 , b = 12 , B = 60 a=11,~b=12,~B=60

LOC b 2 = a 2 + c 2 2 a c cos ( B ) 144 = 121 + c 2 11 c 0 = c 2 11 c 23 c = 11 ± 213 2 c can’t be negative, so ignore the - case ATS 1 2 ( 11 + 213 2 ) ( 11 ) ( 3 2 ) = something ugly we don’t care about 60.96 \begin{aligned} \text{LOC}\longrightarrow b^2&=a^2+c^2-2ac\cos(B)\\ 144&=121+c^2-11c\\ 0&=c^2-11c-23\\ c&=\dfrac{11\pm\sqrt{213}}{2}\\ &\text{c can't be negative, so ignore the - case}\\ \text{ATS}& \Longrightarrow \frac{1}{2}\left(\frac{11+\sqrt{213}}{2}\right)(11)\left(\frac{\sqrt{3}}{2}\right)\\ &=\text{something ugly we don't care about}\\ &\approx 60.96\\ \end{aligned}

We have areas 60.96 > 57.2 > 49.9 > 12.4 60.96>57.2>49.9>12.4

Thus summing the three smallest we get ( 33 3 ) + ( 18 3 + 3 39 ) + ( 18 3 3 39 ) = 69 3 (33\sqrt{3})+(18\sqrt3+3\sqrt{39})+(18\sqrt3-3\sqrt{39})=69\sqrt3

Therefore a b a\sqrt{b}

a + b = 69 + 3 = 72 a+b=69+3=72

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