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March 26th, 2014, 11:48 PM  #1 
Newbie Joined: Mar 2014 Posts: 6 Thanks: 0  Clearing fractions in a quadratic equation
The equation is y =  x^2 / 40 + 31x / 40 + 4/5 I clear the fractions by multiplying through by 40 and obtain a solution of x = 1 and x = 32 for the x intercepts. However when I multiply the equation through by 40 the y intercept becomes 32 and is wrong. I thought I had to multiply both sides by 40 which also means I end up with 40y and a y intercept 40 times larger than it should be. Any help would be appreciated. 
March 27th, 2014, 12:12 AM  #2 
Senior Member Joined: Jun 2013 From: London, England Posts: 1,316 Thanks: 116  Re: Clearing fractions in a quadratic equation
How are you calculating the yintercept?

March 27th, 2014, 12:14 AM  #3 
Newbie Joined: Mar 2014 Posts: 6 Thanks: 0  Re: Clearing fractions in a quadratic equation
by clearing the fractions, so multiplying the whole equation by 40, giving x^2 + 31x + 32

March 27th, 2014, 12:15 AM  #4 
Newbie Joined: Mar 2014 Posts: 6 Thanks: 0  Re: Clearing fractions in a quadratic equation
Then substituting x = 0 into said equation

March 27th, 2014, 12:27 AM  #5  
Senior Member Joined: Jun 2013 From: London, England Posts: 1,316 Thanks: 116  Re: Clearing fractions in a quadratic equation Quote:
 
March 27th, 2014, 12:32 AM  #6 
Newbie Joined: Mar 2014 Posts: 6 Thanks: 0  Re: Clearing fractions in a quadratic equation
The question is y = x^2 / 40 + 31x / 40 + 4/5 I multiply the equation through by 40 which gives: 40(y) = x^2 + 31x + 32 
March 27th, 2014, 12:35 AM  #7 
Newbie Joined: Mar 2014 Posts: 6 Thanks: 0  Re: Clearing fractions in a quadratic equation
If y = 0 then 40 x y = 0 so it is in quadratic format 0 = x^2 + 31x + 32 When I use a graph app, the solution of x is correct x = 1 and x = 32 but the y intercept is wrong, it is not 32 but remains as 4/5 but I do not understand how when your supposed to multiply all of the equation by 40 
March 27th, 2014, 12:51 AM  #8 
Senior Member Joined: Jun 2013 From: London, England Posts: 1,316 Thanks: 116  Re: Clearing fractions in a quadratic equation
If you multiply an equation by 40, you multiply all of an equation by 40. You can't multiply part of an equation by 40 and not the rest of it. So, you multiplied by 40 and got: To find the yintercept, you set x = 0, which gives: Which is exactly the same as putting x = 0 in the original equation. I suspect you're just typing the xexpression into your computer, which is assuming that's the expression for y. But, of course, that's the expression for 40y. The problem is perhaps more understanding the software you're using than the maths! 
March 27th, 2014, 01:05 AM  #9 
Newbie Joined: Mar 2014 Posts: 6 Thanks: 0  Re: Clearing fractions in a quadratic equation
Of course, now you have mentioned that i can see it now. I just haven't seen a quadratic with 40y, only with one y. of course its just a case of rearranging the equation. Thanks for your help, appreciate that.


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clearing, equation, fractions, quadratic 
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