Finding the turning point for the curve f(x)2x^2-6x-8

**agehayoshina** · December 15th 2012, 10:46 AM

I have found out the coordinates are (-1,0)(4,0)(0,-8) but I can't find the turning point

Thanks!

**skeeter** · December 15th 2012, 10:50 AM

the vertex, or 'turning point", of $f(x) = ax^2 + bx + c$ is at $x = \frac{-b}{2a}$

**agehayoshina** · December 15th 2012, 10:54 AM

Hmm this is till kind of confusing. If I plug the numbers in i get -6/2(2) =-1.5. That's x. How do I find y?

Thanks!

**skeeter** · December 15th 2012, 11:53 AM

Originally Posted by agehayoshina

Hmm this is till kind of confusing. If I plug the numbers in i get -6/2(2) =-1.5. That's x. How do I find y?

you have miscalculated the x-value of the vertex.

$f(x) = 2x^2 - 6x - 8$

$x = \frac{-b}{2a} = \frac{-(-6)}{2(2)} = \frac{3}{2}$

the y-value of the vertex is $\color{red}y = f\left(\frac{3}{2}\right)$

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