Are you talking about Vieta theorem of the roots of the quadratic equation? The theorem is a perfect example of the simplest system of two equations with two variables. Let x be the one number, then y would be the other one. The system:
{x + y = -10, xy = -8} Solution: x = -10 - y --> into the 2nd equation --> (-10 - y)y = -8 => y^ + 10y - 8 = 0. D = 10^2/4 + 8 = 25 + 8 = 33. y = -5 +-sqrt(33) --->
x1 = -10 -(- 5 + sqrt(33)) = -5 - sqrt(33), x2 = -10 -( - 5 - sqrt(33)) = - 5 + sqrt(33) solve(x + y = -10, xy = -8, x,y)
However, the solution suggested by web2.0calc looks questionably, particularly regarding the arrangment for y. The formulae for quadratics roots of the reduced qudratic equation of the x^2 + px + q = 0 kind looks as x = -p/2 +-sqrt(D), and the one for D is p^2/4 - q.
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