How many positive integers less than 1,000,000 have exactly one digit equal to 9 and have a sum of digits equal to 13
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Answer:
Step-by-step explanation:
We have that the focus is at (p,0) and that the directrix is x=-p. Take the point (-p,0) of the directrix. We know that the vertex of the parabola is at the middle point ot the line segment that joins the points (p,0) and (-p,0). To get the middle piont, we take the average coordinate by coordinate. That is, the middle point is . The general formula of a parabola of vertex (h,k) that opens to the right or to the left is given by
Where |p| is the distance from the focus to the vertex. If the parabola opens to the right, then p>0 and p<0 otherwise. In our case, h =0=k, so we get that
