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Question

In $\Delta\text{XYZ}$, $\text{XY} = 12\text{ cm}$ and $\text{YZ} = 18\text{ cm}$. $\text{XW}$, the angle bisector of $\text{YXZ}$, meets $\text{YZ}$ at $\text{W}$, such that $\text{YW} : \text{WZ}$ is $4 : 5$. Find the length of the third side of the triangle.

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
$15\text{ cm}$

Triangle Geometry: Angle Bisector Theorem

The problem involves finding the length of the third side of a triangle using the Angle Bisector Theorem.

Given:

  • Triangle $\Delta\text{XYZ}$
  • Side lengths: $\text{XY} = 12\text{ cm}$, $\text{YZ} = 18\text{ cm}$
  • XW is the angle bisector of $\angle\text{YXZ}$, meeting $\text{YZ}$ at W.
  • Ratio: $\text{YW} : \text{WZ} = 4 : 5$
  • To find: Length of side $\text{XZ}$.

Applying the Angle Bisector Theorem

The Angle Bisector Theorem states that an angle bisector of a triangle divides the opposite side into two segments that are proportional to the other two sides of the triangle.

For $\Delta\text{XYZ}$ and angle bisector XW:

$ \frac{\text{YW}}{\text{WZ}} = \frac{\text{XY}}{\text{XZ}} $

Calculation

We are given the ratio $\frac{\text{YW}}{\text{WZ}} = \frac{4}{5}$ and the side length $\text{XY} = 12\text{ cm}$.

Substitute these values into the Angle Bisector Theorem equation:

$ \frac{4}{5} = \frac{12\text{ cm}}{\text{XZ}} $

To find $\text{XZ}$, rearrange the equation:

$ \text{XZ} = \frac{12\text{ cm} \times 5}{4} $

Calculate the result:

$ \text{XZ} = \frac{60\text{ cm}}{4} $ $ \text{XZ} = 15\text{ cm} $

Result

The length of the third side, $\text{XZ}$, is $15\text{ cm}$.

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  2. What is the centroid of the triangle ABC?

  3. What is the foot of the altitude from the vertex A of the triangle ABC?

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