document.write( "Question 982005: Find the standard form of the equation of the parabola with a focus at (-4, 0) and a directrix at x = 4.\r
\n" ); document.write( "\n" ); document.write( " a. y^2 = -8x
\n" ); document.write( " b. 16y = x^2
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\n" ); document.write( "\n" ); document.write( "I tried to solve this problem and got y^2= -16 but thats not an answer choice. Please help! Is it y^2 = -8x? \n" ); document.write( "

Algebra.Com's Answer #602874 by Edwin McCravy(20059) $\"\"$ $\"About$

You can put this solution on YOUR website!
Find the standard form of the equation of the parabola with a focus at (-4, 0) and a directrix at x = 4.
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document.write( "I won't do that exact problem but one exactly like it so you can use it as a \r\n" );
document.write( "model to do yours by.  I'll do this one instead:\r\n" );
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\n" ); document.write( "Find the standard form of the equation of the parabola with a focus at (-3, 0) and a directrix at x = 3.
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document.write( "See the graph below.  I plot the focus, draw the directrix (in green). And\r\n" );
document.write( "since I know that thevertex is halfway between the focus and the directrix, I\r\n" );
document.write( "know that the vertex is (0,0).  I know that the focal distance (from the vertex\r\n" );
document.write( "to the focus is -3 (negative because you have to go left from the vertex to the\r\n" );
document.write( "focus. I also know that a parabola that opens left or right has the equation\r\n" );
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document.write( "(y-k)² = 4p(x-h) \r\n" );
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document.write( "where (h,k) is the vertex and p = the focal distance. So I\r\n" );
document.write( "substitute h=0, k=0, p=-3, and get\r\n" );
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document.write( "(y-0)² = 4(-3)(x-0)\r\n" );
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document.write( "y² = -12x\r\n" );
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document.write( "Now do yours the exact same way.\r\n" );
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document.write( "Edwin

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