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 August 16th, 2013, 10:30 AM #1 Newbie   Joined: Aug 2013 Posts: 4 Thanks: 0 Direct and Inverse Variation Question: {{Studying for the SAT}} The quantities x and y are directly proportional if y=kx for some constant k. For example: x and y are directly proportional. When the value x is 10, y is equal to -5. If x=3, what is y? Because x and y are directly proportional, y=kx for some constant k. You know y is -5 when x is 10; you can use this to find k. y=kx -5=k(10) k= -1/2 <---- this is what i am confused about how am i supposed to know the 10/-5 is supposed to equal -1/2 instead of -2?? I understand that obviously is half of 10 but.... ? Goes on to say: So, the equation is y=-1/2x. When x=3, you get y=(-1/2)(3)= -3/2 Is it supposed to be a fraction for Direct and Inverse Variations? This is what confuses me in math, how i am supposed to know what kind of equation it is and what formula to use...
August 16th, 2013, 10:49 AM   #2
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Re: Direct and Inverse Variation

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 Originally Posted by chelsf123 Question: {{Studying for the SAT}} The quantities x and y are directly proportional if y=kx for some constant k. For example: x and y are directly proportional. When the value x is 10, y is equal to -5. If x=3, what is y? Because x and y are directly proportional, y=kx for some constant k. You know y is -5 when x is 10; you can use this to find k. y=kx -5=k(10)
$-5=k(10)$ is absolutely correct. Tell me what you did to both sides of this equation to find $k$ ?

 August 16th, 2013, 07:40 PM #3 Newbie   Joined: Aug 2013 Posts: 4 Thanks: 0 Re: Direct and Inverse Variation divided both sides by -5 so it would come out to be -2 or -1/2 right so what i didnt understand is why they wrote it as -1/2 instead of -2 do you get what im saying? do you have to use fraction form for the constant?
 August 16th, 2013, 07:41 PM #4 Newbie   Joined: Aug 2013 Posts: 4 Thanks: 0 Re: Direct and Inverse Variation or would you divide both sides by 10?
 August 17th, 2013, 07:53 AM #5 Newbie   Joined: Aug 2013 Posts: 6 Thanks: 0 Re: Direct and Inverse Variation The quantities x and y are directly proportional if y=kx for some constant k. For example: x and y are directly proportional. When the value x is 10, y is equal to -5. If x=3, what is y? Because x and y are directly proportional, y=kx for some constant k. You know y is -5 when x is 10; you can use this to find k. Directly proportional so we use the equation y=kx. -5=k10 --->divide both sides by 10---> k=-1/2 To find y when x=3: y=kx y=(-1/2) * (3) y= -3/2 Hope this helps.
August 17th, 2013, 07:56 AM   #6
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Re: Direct and Inverse Variation

Quote:
 Originally Posted by chelsf123 or would you divide both sides by 10?
You are solving for k. So, you want to get k by itself. If you divide by -5, you won't completely have the K value itself. You want to know the "k" value so you can plug it back into the y=kx equation when you know the given value of x (which equals to 3) so you can solve for y.

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