03 // Bidirectional STEM Calculators // MATHc = √(a² + b² - 2ab cos(C))

Law of Cosines Triangle Side & Angle Calculator

The Law of Cosines generalizes the Pythagorean Theorem to all oblique triangles, relating side c to sides a and b and their included angle C via c² = a² + b² - 2ab cos(C).

c = √(a² + b² - 2ab cos(C))

Pick which value you want to calculate (or type into any field):Solving for: c — Opposite Side c

Opposite Side cCalculated

Side opposite angle C

0.110,000.00

Adjacent Side a

First side flanking angle C

0.110,000.00

Adjacent Side b

Second side flanking angle C

0.110,000.00

Included Angle C

Interior angle opposite side c in degrees

1179

deg

Calculated Result · Opposite Side c (c)

Converted units:

Step-by-Step Calculation

01Canonical Law: c = √(a² + b² - 2ab cos(C))

02Given Inputs (Base Units): a = 5 m, b = 8 m, C (°) = 60

03Solve for c (Opposite Side c): c = 7 m

The Law of Cosines generalizes the Pythagorean Theorem to all oblique triangles, relating side c to sides a and b and their included angle C via c² = a² + b² - 2ab cos(C).

Real-World Reference Values

5-8-7 Eisenstein 60° Triangle5² + 8² - 2(5)(8)(0.5) = 49 → c = 7

Exact integer triangle with a 60° angle

Right Angle Limit (C = 90°)cos(90°) = 0 → c² = a² + b²

Reduces identically to the Pythagorean Theorem

Obtuse Triangle (C > 90°)cos(C) < 0 → c² > a² + b²

Term -2ab cos(C) becomes positive

20-Row Comparison Table

Opposite Side c (c) · Adjacent Side a (a)

Compare across:

Law of Cosines Triangle Side & Angle Calculator — Adjacent Side a → Opposite Side c
#	Adjacent Side a (m)	Opposite Side c (m)	Δ %
01	5 mCurrent	7 m	0.00%
02	72.7256 m	69.0739 m	+886.77%
03	145.3512 m	141.5209 m	+1921.73%
04	217.9768 m	214.0889 m	+2958.41%
05	290.6024 m	286.6861 m	+3995.52%
06	363.228 m	359.2948 m	+5032.78%
07	435.8536 m	431.9091 m	+6070.13%
08	508.4791 m	504.5267 m	+7107.52%
09	581.1047 m	577.1463 m	+8144.95%
10	653.7303 m	649.7673 m	+9182.39%
11	726.3559 m	722.3891 m	+10219.84%
12	798.9815 m	795.0117 m	+11257.31%
13	871.6071 m	867.6348 m	+12294.78%
14	944.2327 m	940.2582 m	+13332.26%
15	1,016.86 m	1,012.88 m	+14369.74%
16	1,089.48 m	1,085.51 m	+15407.23%
17	1,162.11 m	1,158.13 m	+16444.72%
18	1,234.74 m	1,230.75 m	+17482.21%
19	1,307.36 m	1,303.38 m	+18519.70%
20	1,379.99 m	1,376.00 m	+19557.20%

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Frequently Asked Questions

How do I find an unknown angle when all three sides (SSS) are known?+

Click SOLVE FOR on C (°) and enter sides a, b, and c. The engine applies C = arccos((a² + b² - c²) / (2ab)).

When should I use Law of Cosines vs Law of Sines?+

Use Law of Cosines for SAS (two sides and included angle) or SSS (all three sides); use Law of Sines when you have a known angle-side opposite pair.

Why does the Law of Cosines never suffer from the obtuse-angle ambiguity of arcsin?+

Because arccos(x) is strictly one-to-one across the entire triangle angle domain 0° < C < 180°, it uniquely distinguishes acute from obtuse angles.