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the picture).
* Now the triangle is an isosceles triangle with vertex and with base angles .
* Therefore . So and hence is also an isosceles triangle with vertex . The length of its legs is , so the length of is .
* The isosceles triangles and have equal angles of 36° at the vertex, and so they are similar, hence:
* Multiplication with the denominators leads to the quadratic equation:
* This equation for the side

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