What it says
In the chapter on algebra Brahmagupta treats positive numbers as dhana, wealth, and negative numbers as rina, debt. He gives the rules: a debt minus zero is a debt, wealth minus zero is wealth, zero minus zero is zero; the product of two debts is wealth. He also solves quadratic equations and gives a formula for the area of a cyclic quadrilateral.
What it proves
It made zero a number in its own right. Translated into Arabic at Baghdad in the 8th century, it carried Indian mathematics west.
