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April 11th, 2016, 02:52 AM  #1 
Newbie Joined: Apr 2016 From: england Posts: 19 Thanks: 0  a curve to a straight line
hi there I currently have a problem I cant seem to solve y=ax^2/b now the questions asks Imagine that you have been given a series of values for x and corresponding values for y. If you plot y against x your graph will not be a straight line. What graph of y or a term involving y against x or a term involving x could you plot to give a straight line? Note: This part of the question has more than one correct answer, but you are only required to give one of them.? can anyone point me in the right direction please preferably and example that's not like this question so I understand the concept here. 
April 11th, 2016, 02:58 AM  #2 
Senior Member Joined: Jun 2015 From: England Posts: 764 Thanks: 220 
First of all, why not let c = a/b to make the equation easier? Now, have you heard of change of variable by substitution? What would happen if you let $\displaystyle X = {x^2}$ and plotted y against cX? 
April 11th, 2016, 03:09 AM  #3 
Newbie Joined: Apr 2016 From: england Posts: 19 Thanks: 0 
the question is really would that give a straight line or would the answer still come out as a curve ? I can do variable substitution which I overlooked to be honest. thanks 
April 11th, 2016, 03:31 AM  #4 
Newbie Joined: Apr 2016 From: england Posts: 19 Thanks: 0 
this is what I did y = a x^2/b y/x^2 = a/b x^2= a/by x^2=a/b y comparisement with y=mx+c or y = kx gradient =a/b a= gradient/b b= a/gradient 
April 11th, 2016, 03:36 AM  #5 
Newbie Joined: Apr 2016 From: england Posts: 19 Thanks: 0 
I did this as a straight line for non linear equations

April 11th, 2016, 04:00 AM  #6 
Senior Member Joined: Jun 2015 From: England Posts: 764 Thanks: 220 
As I see it, the question only makes sense if you already know x and y. That is you are working on a table of results or measurements from some physical experiment etc. So if I let my c = 1 to remove the constants altogether. Here is a table of such results, what do I get if I plot x^2 against y or y against x^2? 
April 11th, 2016, 04:07 AM  #7 
Newbie Joined: Apr 2016 From: england Posts: 19 Thanks: 0 
your a genius buddy thanks that is what ive been looking for so if a/b =1 then the x and y coordinates would match up so the imputed value of x would ultimately be the corresponding value of y so reall its like y is y^2 as the numbers match 
April 11th, 2016, 04:12 AM  #8  
Math Team Joined: May 2013 From: The Astral plane Posts: 1,689 Thanks: 670 Math Focus: Wibbly wobbly timeywimey stuff.  Quote:
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April 11th, 2016, 04:15 AM  #9 
Newbie Joined: Apr 2016 From: england Posts: 19 Thanks: 0 
thanks there is no negative values but good to point that out when I write it up thankyou 
April 11th, 2016, 05:32 AM  #10 
Senior Member Joined: Jun 2015 From: England Posts: 764 Thanks: 220 
Of course it also depends upon what you mean by plot. As I noted, you have to know both x and y to draw the line on ordinary graph paper, so I don't see a domain issue there. If you only know x to start with, there is no straight line that you can draw connecting x and a non linear function. You can, of course also use non linear axes such as log paper to achieve the straight line objective, but you still have to know x and y to make the plot. However, I did wonder, if you were studying nomograms? With these you would connect suitable non linear axes with straight lines, to read off y, geven x, (as well as a and b). 

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