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FM Cosine Rule

1.5 Methods for Solving Triangles

The 180o Rule

  • For any given triangle, the sum of the inside angles is equal to 180o:

A+B+C=180^{\circ}

Where A, B and C are the inside angles of a triangle.

Example

Picture 1

We wish to find the unknown angle, X, in the above triangle. As the other angles are known, we can use the 180o rule. First, we substitute in the known values:

X+35+45=180

Now we solve for the unknown angle:

X+80 &=180

X &=100^{\circ}

The Sine Rule

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