[Math] Finding A Quadratic Whose Roots Equals Intercept On Axes and Area Equals A

analytic geometryquadratics

How to find the quadratic equation whose roots are the x and y intercepts of the line passing through $(1,1)$ and making a triangle of area A with the axes?

Ok I'm getting $(1-m)(1-1/m)=A$ and $(x-(1-m))(x-(1-1/m))=y$.But how to proceed from here?How to get the sum of roots?

Best Answer

Let's say the quadratic equation is $ax^2+bx+c=0$. Without loss of generality, we assume that $a=1$. Then, the roots are

$$x_{\pm}=\frac{-b\pm\sqrt{b^2-4c}}{2}$$

The product of the roots are simply

$$x_+x_-=c$$

and since we must have $\frac12 x_+x_-=A$, then $c=2A$. Our quadratic equation is now of the form $x^2+bx+2A=0$.

We also know that the line that passes through $(0,x_+)$ and $(x_-,0)$ also passes through $(1,1)$. Thus, we have

$$\frac{x_+-1}{1}=\frac{x_+}{x_-}\implies x_+x_-=x_++x_-\implies c=-b\implies b=-c=-2A$$

Thus, the quadratic equation is

$$\bbox[5px,border:2px solid #C0A000]{x^2-2Ax+2A=0}$$