For 10 points, identify this geometric theorem, named for a Greek mathematician, which gives the area of a triangle in terms of side lengths.
A generalized form of this statement is named for Brahmagupta, and this statement's namesake proved it in Proposition 1.8 of his Metrica. To use it, calculate half of the sum of the side lengths, known as the (*) semiperimeter, to find the difference between the semiperimeter and each side length, then multiply the semiperimeter by the product of those differences, and take the square root. For 10 points, identify this geometric theorem, named for a Greek mathematician, which gives the area of a triangle in terms of side lengths.
A generalized form of this statement is named for Brahmagupta, and this statement's namesake proved it in Proposition 1.8 of his Metrica.
Heron's formula [accept Hero's formula]