document.write( "Question 107873: Suppose you have a lemonade stand, and when you charge $2 per cup of lemonade you sell 120 cups. But when you raise your price to $3 you only sell 60 cups. Write an equation for the number of cups you sell as a function of the price you charge. Denote \"C\" for number of cups, and \"P\" for the price you charge. Assume the function is linear.
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\n" ); document.write( "This is what I have so far, please help!
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\n" ); document.write( "Price- $2, cups sold- 120
\n" ); document.write( "Price- $3, cups sold- 60
\n" ); document.write( "Take 120 from 60 we get a= -60
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Algebra.Com's Answer #78635 by jim_thompson5910(35256)\"\" \"About 
You can put this solution on YOUR website!
Let x=price, y=# of cups sold\r
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\n" ); document.write( "\n" ); document.write( "So when x=2, then y=120 (ie you sell 120 cups at $2). So that means you have the point (2,120). Now let (\"x%5B1%5D\",\"y%5B1%5D\") be that first point (ie \"x%5B1%5D=2\" and \"y%5B1%5D=120\")\r
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\n" ); document.write( "\n" ); document.write( "Also when x=3, then y=60 (ie you sell 60 cups at $3). So that means you have the point (3,60). Now let (\"x%5B2%5D\",\"y%5B2%5D\") be the second point (ie \"x%5B2%5D=3\" and \"y%5B2%5D=60\")\r
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\n" ); document.write( "\n" ); document.write( "So we have two points (2,120) and (3,60)\r
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\n" ); document.write( "\n" ); document.write( "Now let's find the equation of the line through those points\r
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\n" ); document.write( "\n" ); document.write( "First lets find the slope through the points (\"2\",\"120\") and (\"3\",\"60\")\r
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\n" ); document.write( "\n" ); document.write( "\"m=%28y%5B2%5D-y%5B1%5D%29%2F%28x%5B2%5D-x%5B1%5D%29\" Start with the slope formula (note: (\"x%5B1%5D\",\"y%5B1%5D\") is the first point (\"2\",\"120\") and (\"x%5B2%5D\",\"y%5B2%5D\") is the second point (\"3\",\"60\"))\r
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\n" ); document.write( "\n" ); document.write( "\"m=%2860-120%29%2F%283-2%29\" Plug in \"y%5B2%5D=60\",\"y%5B1%5D=120\",\"x%5B2%5D=3\",\"x%5B1%5D=2\" (these are the coordinates of given points)\r
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\n" ); document.write( "\n" ); document.write( "\"m=+-60%2F1\" Subtract the terms in the numerator \"60-120\" to get \"-60\". Subtract the terms in the denominator \"3-2\" to get \"1\"\r
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\n" ); document.write( "\n" ); document.write( "So the slope is\r
\n" ); document.write( "\n" ); document.write( "\"m=-60\"\r
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\n" ); document.write( "\n" ); document.write( "Now let's use the point-slope formula to find the equation of the line:\r
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\n" ); document.write( "\n" ); document.write( "------Point-Slope Formula------
\n" ); document.write( "\"y-y%5B1%5D=m%28x-x%5B1%5D%29\" where \"m\" is the slope, and (\"x%5B1%5D\",\"y%5B1%5D\") is one of the given points\r
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\n" ); document.write( "\n" ); document.write( "So lets use the Point-Slope Formula to find the equation of the line\r
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\n" ); document.write( "\n" ); document.write( "\"y-120=%28-60%29%28x-2%29\" Plug in \"m=-60\", \"x%5B1%5D=2\", and \"y%5B1%5D=120\" (these values are given)\r
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\n" ); document.write( "\n" ); document.write( "\"y-120=-60x%2B%28-60%29%28-2%29\" Distribute \"-60\"\r
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\n" ); document.write( "\n" ); document.write( "\"y-120=-60x%2B120\" Multiply \"-60\" and \"-2\" to get \"120\"\r
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\n" ); document.write( "\n" ); document.write( "\"y=-60x%2B120%2B120\" Add \"120\" to both sides to isolate y\r
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\n" ); document.write( "\n" ); document.write( "\"y=-60x%2B240\" Combine like terms \"120\" and \"120\" to get \"240\"\r
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\n" ); document.write( "\n" ); document.write( "Answer:\r
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\n" ); document.write( "\n" ); document.write( "So the equation of the line which goes through the points (\"2\",\"120\") and (\"3\",\"60\") is:\"y=-60x%2B240\"\r
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\n" ); document.write( "\n" ); document.write( "The equation is now in \"y=mx%2Bb\" form (which is slope-intercept form) where the slope is \"m=-60\" and the y-intercept is \"b=240\"\r
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\n" ); document.write( "\n" ); document.write( "Notice if we graph the equation \"y=-60x%2B240\" and plot the points (\"2\",\"120\") and (\"3\",\"60\"), we get this: (note: if you need help with graphing, check out this solver)\r
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\n" ); document.write( "\n" ); document.write( " Graph of \"y=-60x%2B240\" through the points (\"2\",\"120\") and (\"3\",\"60\")\r
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\n" ); document.write( "\n" ); document.write( "Notice how the two points lie on the line. This graphically verifies our answer.\r
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\n" ); document.write( "\n" ); document.write( "Since the equation is \"y=-60x%2B240\" the function is \"C%28p%29=-60p%2B240\" where \"C%28p%29\" is the cost (relative to a given price) and p is the price
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