[SOLVED] Quadratics/Parabolas Help

**22upon7** · February 23rd 2009, 09:37 PM

Can someone show me the steps of working out:

The X Axis intercepts

If the function of the graph is given.

Or any other websites where I could learn these.

Thanks,

Dru

**Grandad** · February 23rd 2009, 10:07 PM

Hello 22upon7

Originally Posted by 22upon7

Can someone show me the steps of working out:

1. The coordinates of the turning point
2. The Axis of Symmetry
3. The X Axis intercepts

If the function of the graph is given.

Or any other websites where I could learn these.

Thanks,

Dru

I'm assuming that the equation you're referring to is in the form $y = ax^2 +bx + c$ . As an example, I'll use $y = 2x^2 - 7x +3$ . So:

(1) The coordinates of the turning point are found as follows:

Differentiate, and find the value of $x$ for which $\frac{dy}{dx} =0$
Substitute this value of $x$ in the original equation to find the value of $y$

So: $y = 2x^2 - 7x +3$

$\Rightarrow \frac{dy}{dx}=4x -7 =0$ when

$x = \frac{7}{4}$ and $y = 2\times \frac{49}{16}-7\times \frac{7}{4}+3 = -\frac{25}{8}$

So the turning point is $\left(\frac{7}{4}, -\frac{25}{8}\right)$ , and it is a minimum since $\frac{d^2y}{dx^2}=4>0$ .

(2) The equation of the line of symmetry is simply the line through the turning point parallel to the $y$ -axis.

In this example, then, this is $x = \frac{7}{4}$ .

(3) The $x$ -axis intercepts are the values of $x$ for which $y = 0$

So: $y =0$ when $2x^2 -7x +3 = 0$

$\Rightarrow (2x-1)(x-3)=0$

$\Rightarrow x = \tfrac{1}{2}$ or $3$

So the intercepts are $(\tfrac{1}{2}, 0)$ and $(3,0)$ .

Grandad

**22upon7** · February 23rd 2009, 10:20 PM

Originally Posted by Grandad

Hello 22upon7I'm assuming that the equation you're referring to is in the form $y = ax^2 +bx + c$ . As an example, I'll use $y = 2x^2 - 7x +3$ . So:

(1) The coordinates of the turning point are found as follows:

Differentiate, and find the value of $x$ for which $\frac{dy}{dx} =0$
Substitute this value of $x$ in the original equation to find the value of $y$

So: $y = 2x^2 - 7x +3$

$\Rightarrow \frac{dy}{dx}=4x -7 =0$ when

$x = \frac{7}{4}$ and $y = 2\times \frac{49}{16}-7\times \frac{7}{4}+3 = -\frac{25}{8}$

So the turning point is $\left(\frac{7}{4}, -\frac{25}{8}\right)$ , and it is a minimum since $\frac{d^2y}{dx^2}=4>0$ .

(2) The equation of the line of symmetry is simply the line through the turning point parallel to the $y$ -axis.

In this example, then, this is $x = \frac{7}{4}$ .

(3) The $x$ -axis intercepts are the values of $x$ for which $y = 0$

So: $y =0$ when $2x^2 -7x +3 = 0$

$\Rightarrow (2x-1)(x-3)=0$

$\Rightarrow x = \tfrac{1}{2}$ or $3$

So the intercepts are $(\tfrac{1}{2}, 0)$ and $(3,0)$ .

Grandad

Thanks a lot, I understood most of that, the one part I didn't get was the $\frac{dy}{dx}$ . Would appreciate if you could detail that part for me, thanks again.

**Grandad** · February 24th 2009, 12:02 PM

Hello 22upon7

Originally Posted by 22upon7

Thanks a lot, I understood most of that, the one part I didn't get was the $\frac{dy}{dx}$ . Would appreciate if you could detail that part for me, thanks again.

Have you covered differentiation in your course yet? This is an very important topic with many and varied applications, the simplest of which is to find a formula for the gradient of a curve at a particular point.

You'll probably need a teacher to explain it to you, and this forum isn't the place to detail all that you need to know. However, here's a web-site with a summary that you might find helpful: Differentiation

Grandad

**masters** · February 24th 2009, 01:15 PM

Originally Posted by Grandad

Hello 22upon7I'm assuming that the equation you're referring to is in the form $y = ax^2 +bx + c$ . As an example, I'll use $y = 2x^2 - 7x +3$ . So:

Hi 22upon7,

Grandad showed you how to factor and find the x-intercepts.

Let me show you another way to find the vertex (turning point) without differentiation. Your posting history indicates you would like to see an algebraic solution.

If you need to find the vertex of this parabola, first find the x-coordinate of the vertex by adding the x-intercepts and dividing the result by 2. This is the same thing as finding the midpoint between the two x-intercepts. Parabolas are symmetrical, so the axis of symmetry would run through this point.

There's also a generic formula for finding the x-coordinate of the vertex. It's $x=\frac{-b}{2a}$ from the general form $ax^2+bx+c=0$

So using either method, we find the x-coordinate of the vertex to be $\frac{7}{4}$ .

To find the y-coordinate of the vertex, find $f\left(\frac{7}{4}\right)=2x^2-7x+3$

$f\left(\frac{7}{4}\right)=-\frac{25}{8}$

So the vertex of this parabola is $\left(\frac{7}{4}, -\frac{25}{8}\right)$

Thread: [SOLVED] Quadratics/Parabolas Help

LinkBack

Thread Tools

Display

[SOLVED] Quadratics/Parabolas Help

Quadratics/Parabola

Differentiating

Similar Math Help Forum Discussions

[SOLVED] Factorising quadratics

[SOLVED] Quadratics and Co-ordinate Geometry

[SOLVED] Quadratics setting on cacluator

[SOLVED] Finding equation for parabolas, radical function y = root x, asymptotes &amp

[SOLVED] factorising quadratics

Search Tags