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2 answers:
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If each years cumulate an increase of 1100 dollars, then the cost after 22 years will be the base cost (27000) plus 22 increases (22*1100), for a total of
This relates to arithmetic series as follows: a series is arithmetic if, given a starting element , every next element is given by adding a fixed number, called common difference,
. So, the first terms are
So, you can see that the element is obtained by adding n times the common ratio to the starting element.
Since the element represents the cost after n years, we are interested in the element
As we had already calculated.
Check the picture below.
so the focus point is there, and the directrix is above it, meaning is a vertical parabola and is opening downwards, since the parabola opens up towards the focus.
now, the vertex is half-way between those two guys, at a "p" distance from either one, if we move over the y-axis from -5 to +2, we have 7 units, half-way is 3.5 units, and that puts us at -1.5 or -1½, as you see in the picture, so the vertex is then at (-3 , -1½).
so the distance from the vertex to the focus point is then 3½ units, however since the parabola is opening downwards, "p" is negative, thus "p = 3½".
