Triangle Third Side

The Third Side of a Triangle calculator computes the third side of a triangle (c) based on the length of the two other sides (a and b) and the subtended angle (Δ).

INSTRUCTIONS: Choose units and enter the following:

• (a) Length of side a
• (b) Length of side b
• (Δ) Subtended Angle 

Third Side of Triangle (d):  The length is returned in centimetres.  However this can be automatically converted to compatible units via the pull-down menu.

Related Calculators

• Triangle Third Side computes the third side (c) from two sides (a &b) and adjacent angle (A).
• Triangle Angle computes the interior angle (AA), based on the length of the three side (a, b, c)
• Triangle Semi-Perimeter

The Math / Science

The formula the law of sines where:

                   ` a/(sin α) = b / (sin β ) = d / (sin Δ)`

and the fact that the sum of the interior angles is equal to PI radians.

.1 Use Law of Sines and a, b and