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Given a triangle with sides a, b and c and angle X, the Law of Cosines states

c² = a² + b² - (2)(a)(b)(cos X)

Of course, the Pythagorean theorem is a special case of the Law of Cosines because we have a right angle triangle and in this case angle X = 90^o and we know that cos 90^o = 0 and we are left with c² = a² + b²