<Dl> <Dd> c 2 = b 2 + h 2 . (\ displaystyle c ^ (2) = b ^ (2) + h ^ (2).) </Dd> </Dl> <Dd> c 2 = b 2 + h 2 . (\ displaystyle c ^ (2) = b ^ (2) + h ^ (2).) </Dd> <P> Then use the tangent secant theorem (Euclid's Elements: Book 3, Proposition 36), which says that the square on the tangent through a point B outside the circle is equal to the product of the two lines segments (from B) created by any secant of the circle through B. In the present case: BH = BC BP, or </P> <Dl> <Dd> h 2 = a (a − 2 b cos ⁡ γ). (\ displaystyle h ^ (2) = a (a-2b \ cos \ gamma).) </Dd> </Dl>

When do we use the law of cosines