Question

[Lê Ngọc Thùy Chi]

Bài 11. Cho tam giác ABC vuông ở A, đường cao AH, phân giác AD. Gọi I, J lần lượt là các giao điểm các đường phân giác của tam giác ABH, ACH; E là giao điểm của đường thẳng BI và AJ. Chứng minh rằng: a. Tam giác ABE vuông b) IJ vuông góc với AD

Answer 1

[có vào không thì bảo he]

Let's break down the problem step by step:

Step 1:

We are given a right triangle ABC at vertex A, with altitude AH and median AD. We also know that I and J are the points where the medians of triangles ABH and ACH intersect with each other.

Step 2:

Since triangle ABC is a right triangle, we know that angle A is a right angle (90°). Therefore, we can conclude that triangle ABE is also a right triangle (with angle ABE being a right angle).

Step 3:

Now, let's focus on triangle ABH. Since I is the point where the median of triangle ABH intersects with the line segment AB, we know that AI = IB (by definition of median). Similarly, since J is the point where the median of triangle ACH intersects with the line segment AC, we know that AJ = JC (by definition of median).

Step 4:

Using the fact that I and J are on opposite sides of angle ABE, we can write:

AI + IB = AJ + JC

Since AI = IB and AJ = JC, we can simplify this equation to:

2IB = 2JC

Step 5:

Now, let's look at the triangles ABE and ACE. Since they share side AE and angle E is common to both triangles, we can say that:

∠EAB = ∠ECA (common angles)

Using this fact, we can conclude that:

AE = EB (since opposite sides of equal angles are equal)

Step 6:

Now we have:

AE = EB and IB = JC

Using these two equations, we can write:

IJ = IB - JC = AE - AE = 0

So, IJ is a zero-length line segment!

Conclusion:

Since IJ is a zero-length line segment, it means that I and J coincide with each other. This implies that:

IJ ⊥ AD (I and J are collinear with AD)

Therefore, we have shown that triangle ABE is a right triangle and IJ is perpendicular to AD.

Answer:

a. Tam giác ABE vuông b) IJ vuông góc với AD