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3 years ago

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Find the standard form of the equation of the parabola with a focus at (-8, 0) and a directrix at x = 8

1 answer:

il63 [147K]3 years ago

8 0

The best thing to do when finding conic equations from the focus and directrix is to sketch a little graph. When you do this, you can easily identify the vertex right in the middle and can determine if the parabola is vertical or horizontal since it must wrap around the focus. This way you know whether the x or the y is squared. When you sketch the focus and directrix, you will see the vertex is at (0,0) and the parabola is sideways, so the y is squared. now remember your formula is y^2=4px where p is the distance from the vertex to the focus, or you could write x=1/(4p)y^2. Now from 0 to -8 is -8 so that means 4p=-32. This gives you y^2=-32x or x=-1/32y^2. Thsi type of question will also be on your module exam and then the final exam so be sure to practice some:)

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Answer: 4/5=80%

7/8= 87.5%

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7 divided by 8 is 0.875, meaning the percentage is 87.5

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a pyramid whose altitude is 5ft weighs 800lbs. at what distance from its vertex must it be cut by a plane parallel to its base s

slavikrds [6]

A structure which has a square base and four triangular sides meeting at a point is called pyramid.

At distance of 3.97 feet from its vertex , pyramid is cut by plane so that the two solids of equal weight will be formed.

<u>It is assumed that weight of pyramid is proportional to its volume.</u>

So, $w = k V$ , where V is volume of original pyramid and v is volume of small pyramid and k is constant.

Let us consider that at h distance, pyramid is cut from its vertex. So a small pyramid is also formed.

Assume that base area of original pyramid is A and base of small pyramid is a.

Volume of original pyramid is, $V= \frac{1}{3} *A* 5$

So, weight of original pyramid, $W = k *(\frac{1}{3} *A* 5) = 800$

Volume of small pyramid is, $v = \frac{1}{3}* a* h$

So, weight of small pyramid, $w = k*(\frac{1}{3}* a* h)=400$

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So, $\frac{a}{A} = (\frac{h}{5}) ^{2}$

$a = (\frac{h}{5} )^{2}A$

Substituting value of a in equation $k*(\frac{1}{3}* a* h)=400$

So, $k*(\frac{1}{3}* \frac{h^{2} }{25}A * h)=400$

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Since, $(k *\frac{1}{3} *A* 5) = 800$ , substitute in above equation.

So, $800*\frac{h^{3} }{125}=400\\\\h^{3}=\frac{125}{2}\\\\h=\sqrt[\frac{1}{3} ]{62.5}\\\\h=3.97 feet$

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