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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  
Senior Member Joined: Feb 2010 Posts: 711 Thanks: 147  Re: Direct and Inverse Variation Quote:
 
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  
Newbie Joined: Aug 2013 Posts: 6 Thanks: 0  Re: Direct and Inverse Variation Quote:
 

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