2018 AMC 10 A
Complete problem set with solutions and individual problem pages
Farmer Pythagoras has a field in the shape of a right triangle. The right triangle's legs have lengths and units. In the corner where those sides meet at a right angle, he leaves a small unplanted square so that from the air it looks like the right angle symbol. The rest of the field is planted. The shortest distance from to the hypotenuse is units. What fraction of the field is planted? (2018 AMC 10A Problem, Question#23)
- A.
- B.
- C.
- D.
- E.
Let the square have side length . Connect the upperright vertex of square with the two vertices of the triangle's hypotenuse. This divides the triangle in several regions whose areas must add up to the area of the whole triangle, which is .
Square has area , and the two thin triangle regions have area and . The final triangular region with the hypotenuse as its base and height has area . Thus, we have .
Solving gives . The area of is and the desired ratio is .
Alternatively, once you get , you can avoid computation by noticing that there is a denominator of , so the answer must have a factor of in the denominator, which only does.
Let the square have side length . If we were to extend the sides of the square further into the triangle until they intersect on point on the hypotenuse, we'd have a similar right triangle formed between the hypotenuse and the two new lines, and smaller similar triangles that share a side of length . Using the sidetoside ratios of these triangles, we can find that the length of the big similar triangle is . Now, let's extend this big similar right triangle to the left until it hits the side of length . Now, the length is , and using the ratios of the side lengths, the height is . Looking at the diagram, if we add the height of this triangle to the side length of the square, we'd get , so area of square is . Now comes the easy part: finding the ratio of the areas: .
We use coordinate geometry. Let the right angle be at and the hypotenuse be the line for . Denote the position of as , and by the point to line distance formula, we know that . Obviously , so , and from here the rest of the solution follows to get .
Let the side length of the square be . First off, let us make a similar triangle with the segment of length and the topright corner of . Therefore, the longest side of the smaller triangle must be . We then do operations with that side in terms of . We subtract from the bottom, and from the top. That gives us the equation of . Solving, .
Thus, , so the fraction of the triangle (area ) covered by the square is . The answer is then .
On the diagram above, find two smaller triangles similar to the large one with side lengths , , and ; consequently, the segments with length and .
Find an expression for : using the hypotenuse, we can see that .
Simplifying, , or .
A different calculation would yield , so . In other words, , while to check, . As such, , and .
Finally, we get , to finish. As a proportion of the triangle with area , the answer would be , so is correct.
