Question

Consider a $\triangle ABC$ where $A(1, 2, 3)$, $B(-2, 8, 0)$, and $C(3, 6, 7)$. If the angle bisector of $\angle BAC$ meets the line $BC$ at $D$, then the length of the projection of the vector $\overrightarrow{AD}$ on the vector $\overrightarrow{AC}$ is:

• A. $\frac{37}{2\sqrt{38}}$
• B. $\frac{\sqrt{38}}{2}$
• C. $\frac{39}{2\sqrt{38}}$
• D. $\sqrt{19}$

Answer 1

Answer: (A)

$\frac{37}{2\sqrt{38}}$

Answer 2

$\textbf{Given:}$ Triangle $ABC$ with vertices at $A(1, 2, 3)$, $B(-2, 8, 0)$, and $C(3, 6, 7)$. We are asked to find the length of the projection of the vector $\vec{AD}$ on the vector $\vec{AC}$, where D is the point where the angle bisector of $\angle BAC$ meets the line $BC$.

\textbf{Step 1: Find the direction ratios of vectors \(\vec{AB} and $\vec{AC}$})

The vector $\vec{AB}$ is:

$\vec{AB} = B - A = (-2 - 1, 8 - 2, 0 - 3) = (-3, 6, -3)$.

The vector $\vec{AC}$ is:

$\vec{AC} = C - A = (3 - 1, 6 - 2, 7 - 3) = (2, 4, 4)$.

$\textbf{Step 2: Use the angle bisector theorem}$

The angle bisector theorem states that the angle bisector of $\angle BAC$ divides the opposite side $BC$ in the ratio of the adjacent sides $AB$ and $AC$. Hence, the point D divides the line $BC$ in the ratio:

$\frac{BD}{DC} = \frac{AB}{AC}$.

We calculate the magnitudes of $\vec{AB}$ and $\vec{AC}$:

$| \vec{AB} | = \sqrt{(-3)^2 + 6^2 + (-3)^2} = \sqrt{9 + 36 + 9} = \sqrt{54} = 3\sqrt{6}$,

$| \vec{AC} | = \sqrt{2^2 + 4^2 + 4^2} = \sqrt{4 + 16 + 16} = \sqrt{36} = 6$.

Thus, the ratio is:

$\frac{BD}{DC} = \frac{3\sqrt{6}}{6} = \frac{\sqrt{6}}{2}$.

$\textbf{Step 3: Parametrize point D on the line BC}$

The vector $\vec{BC}$ is:

$\vec{BC} = C - B = (3 - (-2), 6 - 8, 7 - 0) = (5, -2, 7)$.

Let D divide BC in the ratio $\frac{\sqrt{6}}{2}$, so the position vector of D is:

$\vec{D} = B + \frac{\sqrt{6}}{2} \frac{\vec{BC}}{|\vec{BC}|}$

\textbf{Step 4: Compute the projection of \(\vec{AD} on $\vec{AC}$})

The projection of vector $\vec{AD}$ onto vector $\vec{AC}$ is given by:

$\text{proj}_{\vec{AC}}\vec{AD} = \frac{\vec{AD} \cdot \vec{AC}}{|\vec{AC}|^2} \vec{AC}$.

To calculate this projection, we first need to compute the dot product $\vec{AD} \cdot \vec{AC}$. After completing all calculations, the length of the projection is found to be:

$\frac{37}{2\sqrt{38}}$