The latus rectum of the parabola with the equation 4x 2 - 20x + 24y + 61 = 0 is

The latus rectum is 6.

We first find the focal point of the function.  To do this, we need the vertex.  We will rewrite this in standard form:

4x²-20x+24y+61=0
24y=-4x²+20x-61
y=(-4/24)x²+(20/24)x-(61/24)
y=(-1/6)x²+(5/6)x-(61/24)

We will factor a, -1/6, out of this:
y=(-1/6)(x²-5x+61/4)

The vertex is found by completing the square.  To do this, we will find b/2 first:
-5/2
Now we square it:
(-5/2)²=25/4
This is what we add in parentheses to our function:
y = (-1/6)(x²-5x+25/4+61/4)
We must also subtract it:
y = (-1/6)(x²-5x+25/4+61/4-25/4)
The last two numbers will be multiplied by the -1/6 to be removed from parentheses:
y = (-1/6)(x²-5x+25/4)-61/24+25/24
y = (-1/6)(x-5/2)² - 36/24
y = (-1/6)(x-5/2)² - 3/2

Now that it is in vertex form, y=a(x-h)²+k, we see that the vertex, (h, k) is at (5/2, -3/2).

To find the focus, the equation needs to be in the form
(x-h)² = 4p(y-k)²

We know that (h, k) is the vertex:
(x-5/2)² = 4p(y--3/2)²
(x-5/2)² = 4p(y+3/2)²

The difference between this equation and our vertex form is that the k value is moved over with the y, and the value of a in front of x is divided to move it to the y side as well.  a = -1/6; dividing by -1/6 is the same as multiplying by -6:

(x-5/2)² = -6(y+3/2)²

This means that 4p = -6, and p = -6/4 = -3/2.  The focus is at (h, k+p):
(5/2, -3/2+-3/2) = (5/2, -6/2) = (5/2, -3).

The focal length is the distance from the focus to the vertex.  We use the distance formula to find this:



The latus rectum is 4 x the focal length:
4*1.5 = 6

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