2018 AMC 10 A

First, we need to realize that the crease line is just the perpendicular bisector of side , the hypotenuse of right triangle . Call the midpoint of point . Draw this line and cal the intersection point with as . Now, is similar to by similarity. Setting up the ratios, we find that . Thus, our answer is .

Note

In general, whenever we are asked to make a crease, think about that crease as a line of reflection over which the diagram is reflected. This is why the crease must be the perpendicular bisector of , because must be reflected onto . (by pulusona)

Use the ruler and graph paper you brought to quickly draw a triangle of any scale (don't trust the diagram in the booklet). Very carefully fold the acute vertices together and make a crease. Measure the crease with the ruler. If you were reasonably careful, you should see that it measures somewhat more than units and somewhat less than units. The only answer choice in range is .

This is pretty much a cop-out, but it's allowed in the rules technically.

Since is a right triangle, we can see that the slope of line is . We know that if we fold so that point meets point the crease line will be perpendicular to and we also know that the slopes of perpendicular lines are negative reciprocals of one another. Then, we can see that the slope of our crease line is .

Let us call the midpoint of point , the midpoint of point , and the crease line . We know that is parallel to and that 's length is . Using our slope calculation from earlier, we can see that . With this information, we can solve for : . We can then use the Pythagorean Theorem to find .

. Thus, our answer is .

Make use of the diagram in Solution . It can be deduced that . Let .

In , . Then also would be .

In , . After some quick math, we get . Solving for will give .

∴.