Maths Help Please

[Trigger]

Hi,

Would someone mind helping me with a maths question I have? It's not a test or anything, it's just something our teacher has given us over the holidays to do. I've looked in my textbook and found nothing on the subject and have searched google only to find someone trying to flog me a programme Basically, the question is:

Factorise y=2x² - 7x -15 which I think I have done fine and got:

(2x + 3)(x - 5)

Then...

The graph of y=2x² - 7x -15 is sketched below:

(Excuse dodgey drawing )

Find the equation of the line of symmetry of this graph...


Anyone have any ideas? It's probably really simple but we haven't covered it yet so I would appreciate any help

Thanks

Ben

 

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its half way between P and Q, like if P was -4 and Q was 6 then the equation of that line would be x=1

Yeah, but there are no values on the axis i've been given- they're just blank and I can't just write 'halfway between P and Q' as the answer lol

Thanks for the reply BTW

Ben

 

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Yeah as anksta has said,

You need to find the points where y=0

so since youve factorised it you should be almost there.

KaHn

Ahhh, I think I understand now. There should be two posible values for y=0 and the line of symmetry will be halfway in between them?

Thanks again

Ben

 

[Trigger]

/edit:- also your factorisation is wrong

How exactly?

y=2x² - 7x -15 --> (2x + 3)(x - 5)

If I multiply the bit in the brackets out I get what I started with

2x multiplied by x = 2x²

3 multiplied by x = 3x

-5 multiplied by 2x = -10x

and +3 multiplied by -5 = -15, so...

2x² + 3x -10x -15 = 2x² -7x -15 doesn't it?

 

[Trigger]

see my ninja edit

I looked at it quickly and weny b^2-4ac<0 hence imaginary but then i remembered the minus sign

KaHn

hmmm, damn my slow keyboard skillz Sorry So would I be right in saying that the line of symmetry is x = -1?

Thanks

Ben

 

[Trigger]

No that is wrong.

You need to look a (2x+3)=0 and (x-5)=0

Then basically from those two equations what does x=?

KaHn

Well that's what I did but wasn't sure whether it was x = 3 and x = -5 or if the 2x in (2x+3) had an effect on the 3 or not? But wouldn't that make it x = 1.5 and x = -5 ?

Thanks

Ben

 

[Trigger]

No im saying that 3.25 is the middle of that parabolic curve, so that is 3.25 further along from x=-1.5 would be the middle so the line of symmetry is x=1.75

KaHn

Ahh, I see now. It was because I was not dividing by 2 before in (2x+3) when I should have done so Anyways, thank you to everyone for their help, you've all been great

Thanks again

Ben