Problem

Find the equation of the parabola described below. Find the two points that define the latus rectum, and graph the equation.

Focus at (4,7); directrix the line y=5.

Answer

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Answer

So, the equation of the parabola is (x+4)2=4(y6), and the two points that define the latus rectum are (6,7) and (2,7).

Steps

Step 1 :Given that the focus is at (4,7) and the directrix is the line y=5, we can find the vertex of the parabola.

Step 2 :The vertex is the midpoint between the focus and the directrix. So, the y-coordinate of the vertex is the average of the y-coordinate of the focus and the y-coordinate of the directrix, which is (7+5)/2=6. The x-coordinate of the vertex is the same as the x-coordinate of the focus, which is -4. So, the vertex of the parabola is (4,6).

Step 3 :The distance from the vertex to the focus or the vertex to the directrix is p. In this case, p is the difference between the y-coordinate of the focus and the y-coordinate of the vertex, which is 76=1.

Step 4 :Substituting these values into the formula for a vertical parabola, we get (x(4))2=41(y6), which simplifies to (x+4)2=4(y6).

Step 5 :The latus rectum of a parabola is a line segment perpendicular to the axis of symmetry, passing through the focus, and its endpoints lie on the parabola. The length of the latus rectum is 4p=41=4. Since the parabola is vertical, the latus rectum is horizontal. The midpoint of the latus rectum is the focus, so the endpoints of the latus rectum are (42,7) and (4+2,7), which are (6,7) and (2,7).

Step 6 :So, the equation of the parabola is (x+4)2=4(y6), and the two points that define the latus rectum are (6,7) and (2,7).

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