We want to construct a trianlge A B C satisfying the given conditions: A = 3 0 ∘ , A B = 9 5 , B C = 5 5 .
How many such triangles A B C exist up to congruence?
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Another possible approach is to use Law of Cosines:
B C 2 = A B 2 + A C 2 − 2 A B ⋅ A C cos ( ∠ A ) 5 5 2 = 9 5 2 + A C 2 − 2 ( 9 5 ) ( A C ) cos ( 3 0 ∘ ) A C 2 − 9 5 3 A C + 6 0 0 0 = 0
If we compute the discriminant of that quadratic we get:
Δ = ( − 9 5 3 ) 2 − 4 ( 1 ) ( 6 0 0 0 ) = 3 0 7 5
We got that Δ > 0 , so there are 2 distinct real solutions for A C , and by Descartes Rule of Signs, both are positive.
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Let's go back to school and treat this as a construction problem ;)
We can draw the line segment AB = 95, and we draw a ray R emanating from A, making an angle of 3 0 o with AB, there C will be placed. The distance between R and B is sin ( 3 0 o ) × 9 5 = 4 7 . 5 , so that the circle about B with radius 55 will intersect R twice (since 5 5 < 9 5 ), giving us our 2 solutions C.