Cosine and Sine Rule The cosine rule can be considered to be a generalization of the Pythagorean Theorem. Using the Figure 2: Triangle with notations notations from Fig. 2 the rule is: a = b2 + c2 2bc cos a Use a symmetry argument to find two other expressions of this rule. Prove the cosine rule. Hint: split the triangle into two right-angled ones; use the Pythagorean Theorem. While you're at it, the sine rule is: sin a/a = sin/b = sin y/c Prove the sine rule