# Math Help - onto function

1. ## onto function

A function f is from R-------R such that f(x)=(a x^2+6x-8)/(a+6x-8x^2)
find the value of a for which for which f is onto.

A function f is from R-------R such that f(x)=(a x^2+6x-8)/(a+6x-8x^2) find the value of a for which for which f is onto.

$f(x)=y\Leftrightarrow \ldots \Leftrightarrow (a+8y)x^2+(6-6y)x-8-ay=0$

The above equation on $x$ has real solution iff:

$p_a(y)=\Delta=B^2-4AC=\ldots =(9+8a)y^2+(46+a^2)y+9+8a\geq 0$

We have a family of parabolas (a line if $a=-9/8$). We need to find $a$ such that $p_a(y)\geq 0$ for all $y\in\mathbb{R}$ . Necessarily $a>-9/8$ (why?) .

If $a>-9/8$ then, $f$ is onto iff $p(y_0)\geq 0$ being $y_0$ the "abscissa" of the $p_a$ vertex.

Let us see what do you obtain.

3. good evening dear Sir
sir the point you mention that a>-9/8 means a upward parabola .definitely it has a bend some where .so that will give some minimum value of y.sir i am little bit confused here.

good evening dear Sir
sir the point you mention that a>-9/8 means a upward parabola .definitely it has a bend some where .so that will give some minimum value of y.sir i am little bit confused here.

And if that minimum is $\geq 0$ then, $p_a(y)\geq 0$ for all $y\in\mathbb{R}$ .