Math CentralQuandaries & Queries


From Rita, a student:

If a rock falls from a height of 20 meters on the planet Jupiter, its height H
(in meters) after x seconds is about H(x) = 20 - 13x2

(a) What is the height of the rock when x = 1.2 seconds?

(b) When is the height of the rock 15 meters?

(c) When does the rock strike the ground?

Hi Rita,

Part a) is fairly simple, just substitute 1.2 for x into the given equation. Part b) & c) can be solved using the same method. For example, in part b) H(x)=15 so if we substitute it into the original equation: 15= 20 - 13x2 which simplifies to 0= 5 - 13x2. The easiest way to find the time would be to solve for x using the quadratic formula.

For a review on using the quadratic formula check out this review.


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