2015 AMC 10A Problems/Problem 19
The isosceles right triangle has right angle at and area . The rays trisecting intersect at and . What is the area of ?
can be split into a right triangle and a right triangle by dropping a perpendicular from to side . Let be where that perpendicular intersects .
Because the side lengths of a right triangle are in ratio , .
Because the side lengths of a right triangle are in ratio and + , .
Setting the two equations for equal to each other, .
Solving gives .
The area of .
is congruent to , so their areas are equal.
A triangle's area can be written as the sum of the figures that make it up, so .
Solving gives , so the answer is
The area of is , and so the leg length of is Thus, the altitude to hypotenuse , , has length by right triangles. Now, it is clear that , and so by the Exterior Angle Theorem, is an isosceles triangle. Thus, by the Half-Angle formula, and so the area of is . The answer is thus
Because the area of triangle is , and the triangle is right and isosceles, we can quickly see that the leg length of the triangle is 5. If we put the triangle on the coordinate plane, with vertex at the origin, and the hypotenuse in the first quadrant, we can use slope-intercept form and tangents to get three lines that intersect at the origin, , and . Then, you can use the distance formula to get the length of . The height is just , so the area is just
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