Find all possible areas of a triangle with sides of lengths and and one angle.
If the sum of the areas of the smallest triangles is where and are integers and is square-free, enter your answer as .
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Through this solution, I will refer to law of cosines a 2 = b 2 + c 2 − 2 b c cos ( A ) as LOC and area of a triangle in terms of sine 2 1 a b sin C as ATS. Also, lower case letters represent sides while uppercase letters represent angles.
Begin with the simple case of a = 1 1 , b = 1 2 C = 6 0 .
ATS ⟹ 2 1 ( 1 1 ) ( 1 2 ) ( 2 3 ) = 3 3 3 ≈ 5 7 . 1 5 7
Now if a = 1 1 , b = 1 2 , A = 6 0
LOC ⟶ a 2 1 2 1 0 c ATS = b 2 + c 2 − 2 b c cos ( A ) = 1 4 4 + c 2 − 1 2 c = c 2 − 1 2 c + 2 3 = 6 ± 1 3 ⟹ 2 1 ( 6 ± 1 3 ) ( 1 2 ) ( 2 3 ) = 1 8 3 ± 3 3 9 ≈ 4 9 . 9 , 1 2 . 4
Now if a = 1 1 , b = 1 2 , B = 6 0
LOC ⟶ b 2 1 4 4 0 c ATS = a 2 + c 2 − 2 a c cos ( B ) = 1 2 1 + c 2 − 1 1 c = c 2 − 1 1 c − 2 3 = 2 1 1 ± 2 1 3 c can’t be negative, so ignore the - case ⟹ 2 1 ( 2 1 1 + 2 1 3 ) ( 1 1 ) ( 2 3 ) = something ugly we don’t care about ≈ 6 0 . 9 6
We have areas 6 0 . 9 6 > 5 7 . 2 > 4 9 . 9 > 1 2 . 4
Thus summing the three smallest we get ( 3 3 3 ) + ( 1 8 3 + 3 3 9 ) + ( 1 8 3 − 3 3 9 ) = 6 9 3
Therefore a b
a + b = 6 9 + 3 = 7 2