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

c^{2} = a^{2} +
b^{2} - (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
c^{2} = a^{2} +
b^{2}

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