Isosceles Triangle Median: Proving BAD = DAC Angle Equality

Triangle Properties with Median Angle Bisection

Triangle ABC is isosceles (AB=AC).

AD is the median.

Is it true that BAD=DAC ∢\text{BAD}=∢\text{DAC} ?

AAABBBCCCDDD

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Determine whether the angle BAD is equal to the angle DAC
00:03 The following is an isosceles triangle
00:09 AD is a median according to the given information, a median intersects the side
00:12 In an isosceles triangle, the median is also the height
00:20 The height creates a right angle with the side it meets
00:25 In an isosceles triangle, the median is also an angle bisector
00:34 This is the solution

Step-by-step written solution

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1

Understand the problem

Triangle ABC is isosceles (AB=AC).

AD is the median.

Is it true that BAD=DAC ∢\text{BAD}=∢\text{DAC} ?

AAABBBCCCDDD

2

Step-by-step solution

To solve this problem, we need to examine the properties of an isosceles triangle where the median is drawn from the vertex to the base.

Given ABC \triangle ABC is isosceles with AB=AC AB = AC , and AD AD is the median to the base BC BC , it follows certain properties specific to isosceles triangles:

  • In an isosceles triangle, the median from the vertex angle is also the altitude and the angle bisector for the angle at the vertex.
  • Since AD AD is the median, BD=DC BD = DC .
  • This particular property of isosceles triangles means that AD AD not only divides the base BC BC equally but also divides the vertex angle BAC\angle BAC equally, thus forming the angle bisector.

Therefore, the angle BAD\angle BAD is equal to DAC\angle DAC because AD AD serves as the angle bisector of BAC\angle BAC in the isosceles triangle ABC \triangle ABC .

Thus, it is true that BAD=DAC \angle BAD = \angle DAC in this triangle.

Yes.

3

Final Answer

Yes.

Key Points to Remember

Essential concepts to master this topic
  • Isosceles Rule: In isosceles triangles, median from vertex equals angle bisector
  • Technique: Since AB = AC, median AD creates BAD=DAC \angle BAD = \angle DAC
  • Check: Verify BD = DC and both angles are equal ✓

Common Mistakes

Avoid these frequent errors
  • Assuming all triangle medians are angle bisectors
    Don't assume every median bisects angles = wrong conclusions! This special property only works in isosceles triangles when the median is drawn from the vertex angle. Always check if the triangle is isosceles first.

Practice Quiz

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Is the straight line in the figure the height of the triangle?

FAQ

Everything you need to know about this question

Why does the median become an angle bisector in isosceles triangles?

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In an isosceles triangle, the median from the vertex angle has a special property - it's also the altitude, angle bisector, and perpendicular bisector all in one! This happens because of the triangle's symmetry.

Does this work for any median in an isosceles triangle?

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No! This only works for the median drawn from the vertex angle (the angle between the two equal sides) to the base. Medians from the base angles don't have this property.

How can I prove the angles are actually equal?

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Use triangle congruence! Since AB = AC, AD = AD (reflexive), and BD = DC (median property), triangles ABD and ACD are congruent by SSS, making the angles equal.

What if the triangle looks isosceles but isn't given as isosceles?

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Be careful! Always check if AB = AC is explicitly stated or can be proven. If it's not an isosceles triangle, the median won't necessarily bisect the angle.

Is there a way to measure this on the diagram?

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  • Check that AB and AC look equal
  • Verify that D is the midpoint of BC
  • The angles BAD \angle BAD and DAC \angle DAC should look equal

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