# Least possible length

Gold Member

## Homework Statement

If abscissa and ordinate of vertex of parabola $y^2+2ax+2by-1=0$ are equal then least possible length of latus rectum is

## The Attempt at a Solution

The give equation can be rewritten as

$\left( y+b \right) ^2 = -2a\left\{ x-\dfrac{1+b^2}{2a} \right\}$

As given in question

$-b=\dfrac{1+b^2}{2a} \\ 1+b^2+2ab=0$

Length of latus rectum = a/2
But what can be the range of values of a?

Saitama

## Homework Statement

If abscissa and ordinate of vertex of parabola $y^2+2ax+2by-1=0$ are equal then least possible length of latus rectum is

## The Attempt at a Solution

The give equation can be rewritten as

$\left( y+b \right) ^2 = -2a\left\{ x-\dfrac{1+b^2}{2a} \right\}$

As given in question

$-b=\dfrac{1+b^2}{2a} \\ 1+b^2+2ab=0$

Length of latus rectum = a/2
But what can be the range of values of a?

You have "a" as a function of b, how about using that?

Homework Helper
hi utkarshakash! $1+b^2+2ab=0$

well, a can't be 0, can it? have you tried completing the square?​

Gold Member
hi utkarshakash! well, a can't be 0, can it? have you tried completing the square?​

(1+b)^2+2b(a-1)=0

But how does this help?