document.write( "Question 1032247: 1. Use the focus-directrix definition of a parabola to answer the following questions.\r
\n" ); document.write( "\n" ); document.write( "1) How would the shape of the parabola change if the focus were moved up, away from the directrix? How would we describe p?
\n" ); document.write( "2) How would the shape of the parabola change if the focus were moved down, toward the directrix? How would we describe p?
\n" ); document.write( "3) How would the shape of the parabola change if the focus were moved down, below the directrix?\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( " \r
\n" ); document.write( "\n" ); document.write( "2. Given:\r
\n" ); document.write( "\n" ); document.write( "y = 3x^2\r
\n" ); document.write( "\n" ); document.write( "1) Write the equation of a parabola that contains the point (2, ‒12) that is congruent to the parabola that is given. Describe the series of transformations that would move the given parabola to your parabola.\r
\n" ); document.write( "\n" ); document.write( "2) Write the equation of a parabola that contains the point (0, 8) that is congruent to the parabola that is given. Describe the series of transformations that would move the given parabola to your parabola.\r
\n" ); document.write( "\n" ); document.write( "3) Write the equation of a parabola that is similar (not congruent) to the given parabola that does NOT contain the point (0, 0), but does contain the point (2, 2).\r
\n" ); document.write( "
\n" ); document.write( "\n" ); document.write( "Note: Please show all work so that I can understand how to do the problem. Thank you.
\n" ); document.write( "
\n" ); document.write( "

Algebra.Com's Answer #647068 by KMST(5328)\"\" \"About 
You can put this solution on YOUR website!
PROBLEM !:
\n" ); document.write( "1. The focus-directrix definition of a parabola with axis parallel to the y-axis and vertex at the origin leads to the equation
\n" ); document.write( "\"y=%281%2F4p%29x%5E2\"
\n" ); document.write( "for a parabola with directrix \"y=-p\" and focus at \"%22%28+0+%2C+p+%29%22\" .
\n" ); document.write( "It is valid (and useful) to think of the problem based on such a parabola, with \"p%3E0\" parabola, because all other parabolas are variations, flipped and/or translated, and conclusions that works for one works for all (with the adaptations dictated by the flipping and translations used.\r
\n" ); document.write( "\n" ); document.write( "1) Moving the focus \"up, away from the directrix\" means that
\n" ); document.write( "away from the directrix was up, meaning that \"p%3E0\" to begin with, and that the parabola opens up.
\n" ); document.write( " The vertex of the parabola is midway between focus F and directrix, at a distance \"p\" form both. The focus is at a distance \"2p\" from the directrix. A horizontal line through F intersects the parabola at P, which is at a distance \"2p\" from the directrix and Focus F. Focus F and point P are vertices of a square, with sides on the parabola's axis, and its directrix.
\n" ); document.write( "As we move F up, the square expands, and the parabola opes wider:
\n" ); document.write( " You could say the parabola gets compressed vertically, gets flatter, or that it gets stretched horizontally.
\n" ); document.write( "How would we describe \"p\" ? It is the distance between the focus and the vertex of the parabola. In the problem, \"p%3E0\" , and as we move the focus \"up and away\" from the directrix, \"p\" increases.
\n" ); document.write( "
\n" ); document.write( "2) How would the shape of the parabola change if the focus were moved down, toward the directrix? How would we describe p?
\n" ); document.write( "We reverse the motion, from the blue parabola to the green one.
\n" ); document.write( "The parabola gets \"taller, stretched vertically, hugging its axis tighter, and \"p\" decreases.\r
\n" ); document.write( "\n" ); document.write( "3) How would the shape of the parabola change if the focus were moved down, below the directrix?
\n" ); document.write( "You have to jump over the directrix, of course. The focus cannot be on the directrix, with \"p=0\" . As you get the focus down, closer and closer to the directrix, \"p\" gets smaller, and smaller, closer to zero. At some point, \"p\" is incredibly small, the focus is a hair above the directrix, and the parabola has hugged the positive y-axis so tight that you do not see the separation. At that point, you blink and then see that the focus is a hair below the directrix. By then, \"p\" is a negative but almost zero value, the parabola is tightly hugging the negative y-axis. As the focus keeps moving down, away from the directrix, the parabola opens up hugging the negative y-axis less tightly.
\n" ); document.write( "You have flipped the parabola upside down, so it would be a frown rather than a smile. It now open downs, with a \"p%3C0\" , and the more you move the focus away from the directrix the parabola opens wider.
\n" ); document.write( "
\n" ); document.write( "PROBLEM 2:
\n" ); document.write( "A generic parabola, with vertex at the origin, axis on the y-axis, and opening up, with focus \"F%280%2Cp%29\" , with \"p%3E0\" has the equation
\n" ); document.write( "\"y=%281%2F4p%29x%5E2\" .
\n" ); document.write( "Changing the sign of \"p\" , to get \"p%3C0\" would flip the parabola upside down. We could call the flipped parabola congruent, because we can superimposing it by rotating the graph around the vertex of the parabola (the origin).
\n" ); document.write( "Horizontal and/or vertical translations of a parabola, with vertex at the origin, and axis on the y-axis would yield other parabolas that we would call \"congruent\" to each other, because we can superimpose them by using those translations.
\n" ); document.write( "Those parabolas have an equation of the form
\n" ); document.write( "\"y=%281%2F4p%29%28x-h%29%5E2%2Bk\" ,
\n" ); document.write( "where \"h\" is the horizontal rightwards translation,
\n" ); document.write( "and \"k\" is the vertical upwards translation.
\n" ); document.write( "
\n" ); document.write( "\"y=3x%5E2\" is the equation of a parabola with vertex at the origin, axis on the y-axis, and opening up, with focus above the origin, with \"p%3E0\" .
\n" ); document.write( "So, for \"y=3x%5E2\" , \"1%2F4p=3\" .
\n" ); document.write( "1) We can get \"a parabola that contains the point (2, ‒12) that is congruent to the parabola that is given\" by translating the vertex to the point (2, ‒12) .
\n" ); document.write( "That means \"1%2F4p=3\" , \"h=2\" , and \"k=-12\" ,
\n" ); document.write( "which would give us
\n" ); document.write( "\"y=3%28x-2%29%5E2%2B-12\" .
\n" ); document.write( "That parabola could be obtained easily by
\n" ); document.write( "translating \"y=3x%5E2\" 2 units to the left and 12 units down.
\n" ); document.write( "We can find an unlimited number of different examples of \"a parabola that contains the point (2, ‒12) that is congruent to the parabola that is given,\" but that would require more computation.
\n" ); document.write( "For parabolas opening up, they would need to
\n" ); document.write( "have an equation of the form \"y=3%28x-h%29%5E2%2Bk\" with
\n" ); document.write( "\"h\" and \"k\" must be such that they satisfy
\n" ); document.write( "\"-12=3%282-h%29%5E2%2Bk\" .
\n" ); document.write( "
\n" ); document.write( "2) Similarly, \"y=3x%5E2%2B8\"
\n" ); document.write( "is \"a parabola that contains the point (0, 8) that is congruent to the parabola that is given\". translating up by 8 units \"would move the given parabola to\" \"y=3x%5E2%2B8\" .
\n" ); document.write( "
\n" ); document.write( "3) All parabolas are similar, meaning that if you change the scale factor
\n" ); document.write( "(making the units on both axes larger or smaller by the same factor),
\n" ); document.write( "you can superimpose them.
\n" ); document.write( "\"y=%28x-2%29%5E2%2B2\" is \"a parabola that is similar (not congruent) to\" \"y=x3x%5E2\" .
\n" ); document.write( "Since \"y=%28x-2%29%5E2%2B2%3E=2\" , the graph is above the x-axis, and \"does NOT contain the point (0, 0), but does contain the point (2, 2)\".
\n" ); document.write( "Using the same scale, you see that they are clearly not congruent:
\n" ); document.write( " \"%22%2B%22\" \"%22=%22\" .
\n" ); document.write( "However, when we scale down appropriately, we see that they would superimpose:
\n" ); document.write( "\"graph%28300%2C300%2C-4%2F3%2C6%2F3%2C-1%2C7%2F3%2C3x%5E2%29\" \"%22%2B%22\" \"%22=%22\" -->
\n" ); document.write( "
\n" );