Chapter 5 - Polynomials and Factoring - Review Exercises: Chapter 5 - Page 369: 8

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The Pythagorean theorem can only be applied to right triangles. Thus, even for triangles with angles only less than 90 degrees, it is not valid to apply the Pythagorean theorem. Other methods such as the Law of Sines and the Law of Cosines will be learned later on to solve triangles that are not right triangles.