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 Post subject: quadratic functions in standard form (completing square)Posted: Wed, 25 Apr 2012 21:38:13 UTC

Joined: Tue, 24 Apr 2012 03:31:16 UTC
Posts: 6
Write the quadratic function in standard form. Identify vertex, axis of symmetry and sketch graph.
y= -x^2+16x-5

standard form: f(x)= -x^2+16x-5
= -(x^2+16x+64-64)-5
= -(x^2+16x+64)-64 -5
Here is where I get confused...
= -(x^2+16x+64) -69 (?)
=-(x+8)(x+8)-69
multiply thru to rid the - sign out front
= (x-8)^2+69

However according to the markings on my test it should be
=(x-8)^2+59
Which would mean the Vertex is (8,59)
axis:x=8
graph...yadda yadda..... is it really 59? Just wondering where exactly I screwed up here. lol

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 Post subject: Re: quadratic functions in standard form (completing square)Posted: Wed, 25 Apr 2012 22:18:11 UTC
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Joined: Wed, 30 Mar 2005 04:25:14 UTC
Posts: 12103
Location: Austin, TX
stephanini wrote:
Write the quadratic function in standard form. Identify vertex, axis of symmetry and sketch graph.
y= -x^2+16x-5

standard form: f(x)= -x^2+16x-5
= -(x^2+16x+64-64)-5 This is where it goes wrong
= -(x^2+16x+64)-64 -5
Here is where I get confused...
= -(x^2+16x+64) -69 (?)
=-(x+8)(x+8)-69
multiply thru to rid the - sign out front
= (x-8)^2+69

However according to the markings on my test it should be
=(x-8)^2+59
Which would mean the Vertex is (8,59)
axis:x=8
graph...yadda yadda..... is it really 59? Just wondering where exactly I screwed up here. lol

Yes, when you factored out the minus sign, you didn't turn anything but the to the opposite sign. What you should have is

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 Post subject: Re: quadratic functions in standard form (completing square)Posted: Wed, 25 Apr 2012 22:41:40 UTC

Joined: Tue, 24 Apr 2012 03:31:16 UTC
Posts: 6
ohh ok I hate negatives they always mess me up.
Thank you!

got another one coming shortly if ya wanna figure that one out too

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 Post subject: Re: quadratic functions in standard form (completing square)Posted: Wed, 25 Apr 2012 22:43:23 UTC
 Moderator

Joined: Wed, 30 Mar 2005 04:25:14 UTC
Posts: 12103
Location: Austin, TX
stephanini wrote:
ohh ok I hate negatives they always mess me up.
Thank you!

got another one coming shortly if ya wanna figure that one out too

Be sure to show your work again and post it in another topic, and I will take a look.

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