1999 AMC 8 Problems/Problem 25
Points , , and are midpoints of the sides of right triangle . Points , , are midpoints of the sides of triangle , etc. If the dividing and shading process is done 100 times (the first three are shown) and , then the total area of the shaded triangles is nearest
Since is fairly small relative to the rest of the diagram, we can make an underestimate by using the current diagram. All triangles are right-isosceles triangles.
The sum of the shaded regions is
is an underestimate, as some portion (but not all) of will be shaded in future iterations.
If you shade all of , this will add an additional to the area, giving , which is an overestimate.
Thus, is the only answer that is both over the underestimate and under the overestimate.
In iteration , congruent triangles and are created, with one of them being shaded.
In iteration , three more congruent triangles are created, with one of them being shaded.
As the process continues indefnitely, in each row, of each triplet of new congruent triangles will be shaded. The "fourth triangle" at the top ( in the diagram) will gradually shrink,
leaving about of the area shaded. This means square units will be shaded when the process goes on indefinitely, giving .
Using Solution 1 as a template, note that the sum of the areas forms a geometric series:
This is the sum of a geometric series with first term and common ratio This is the easiest way to do this problem.
The sum of an infinite geometric series with is shown by the formula. Insert the values to get , giving an answer of .
Find the area of the bottom triangle, which is 4.5. Notice that the area of the triangles is divided by 4 every time. 4.5*5/4≈5.7, and 5.7+(1/4)≈5.9. We can clearly see that the sum is approaching answer choice .
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