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May 18th, 2015, 06:53 AM  #1 
Member Joined: Sep 2014 From: Sweden Posts: 94 Thanks: 0  Find "Focus and Control Line of Parable"
So I have this equation in my math book. $\displaystyle (x^2)/2 $ This is the "orginal" formula for a parable. Focus: (0, a) Control line: y = a $\displaystyle (x^2)/4a $ They want me to find the focus and the control line of the parable. I have found two coordinates that the parable will hit and that is: ( 2, 2 ) and ( 4, 8 ) But I have no idea of how to find the focus and control line . . . Thanks for help! 
May 18th, 2015, 07:36 AM  #2  
Banned Camp Joined: Jun 2014 From: Earth Posts: 945 Thanks: 191  Quote:
y = (x^2)/2 = (x^2)/(4a) So set 2 = 4a and solve for a: 2 = 4a 1/2 = a You stated your focus is (0, a) and your control line (directrix) is y = a. Now make the substitutions with the avalue.                                              Here are some Internet sites that discuss the vertex, focus, and directrix (what you called a "control line") for a parabola: https://www.google.com/#q=parabola+focus+directrix . Last edited by Math Message Board tutor; May 18th, 2015 at 07:40 AM.  
May 18th, 2015, 07:58 AM  #3 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,299 Thanks: 1023 
y = (x^2)/(4a) y = x^2 / (4a) is sufficient. 
May 18th, 2015, 08:13 AM  #4 
Member Joined: Sep 2014 From: Sweden Posts: 94 Thanks: 0 
I'm sorry about that I use wrong words for some mathematical things. But it's a huge difference between what we call these things in Swedish and in English. But thanks for help, that was the answer I was looking for EDIT: How would I find the focus if I weren't given it (Focus: (0, a)) or is this the formula that is right for all parabolas? Or how would I otherwise do? Last edited by DecoratorFawn82; May 18th, 2015 at 08:36 AM. 
May 18th, 2015, 10:03 AM  #5 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,299 Thanks: 1023  I am officially "scolding" you for your ignorance. You are confusing students with redundant brackets. If your explanation for the above goof is to add clarity, then this is required: y = (x^2)/(2). I'll be reporting this post of yours to the STD (Suspected Trolls Department). You're in trouble deep: the STD CEO is none other than Sir Jonah The Guzzler 
May 19th, 2015, 04:09 AM  #6 
Member Joined: Sep 2014 From: Sweden Posts: 94 Thanks: 0 
What?! I try to explain as well as I can. And I try to write the words correct words in mathematics in English. I'm writing as well as I can and I try to explain what I mean. I don't really get why you would report my posts? I'm only asking for help because I don't know how to solve these problems on my own. And no I'm not trying to troll someone. My posts are serious and I will try to do better in my explanations then. I'll do as good as I can. 
May 19th, 2015, 05:15 AM  #7 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,299 Thanks: 1023 
That was NOT addressed to you...and is only a joke...OK? You're doing quite good for someone learning English. 
May 19th, 2015, 05:29 AM  #8 
Member Joined: Sep 2014 From: Sweden Posts: 94 Thanks: 0 
I didn't see that at first. Okay. Thank you then! 

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