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Law of Sines Triangle Solver

Solve an oblique triangle from two angles and a side (AAS) or two sides and a non-included angle (SSA) using the Law of Sines — including the classic ambiguous case.

Side a
10
Side b
17.3205
Side c
20
Angle A
30°
Angle B
60°
Angle C
90°
Area
86.6025

How it works

The Law of Sines states that a/sin(A) = b/sin(B) = c/sin(C) for any triangle, relating each side to the sine of its opposite angle. Given two angles and a side (AAS), the third angle follows from the angle sum (180° − A − B), and the other two sides follow directly from the ratio — this always has exactly one solution.

Given two sides and a non-included angle (SSA) instead — the angle isn't between the two known sides — the same ratio can produce zero, one, or two valid triangles, historically called the 'ambiguous case'. This calculator solves for both possible values of the unknown angle (asin gives an angle and its supplement) and reports every one that actually forms a valid triangle, rather than silently picking just one.

This is a distinct question from this site's triangle-oblique calculator, which solves SSS and SAS triangles via the Law of Cosines — neither of those two 'given' shapes is AAS or SSA, and only the SSA case here can have more than one correct answer for the same input.

FAQ

Why does the SSA case sometimes give two answers?

Because knowing two sides and an angle not between them doesn't pin down the triangle uniquely — swinging the side opposite the known angle can meet the third side at two different points, each forming a valid triangle. The calculator checks both possibilities from the Law of Sines and keeps whichever ones satisfy the triangle's angle sum.

When does SSA have no solution at all?

When the known side opposite the given angle is too short to reach the third side no matter how it swings — algebraically, when the Law of Sines ratio for the unknown angle exceeds 1 (an impossible sine value). The calculator flags this case explicitly instead of returning a wrong or undefined result.

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