# Find the length of the angle bisector

• Aug 3rd 2012, 05:40 AM
shiny718
Find the length of the angle bisector
ABC is a triangle with AB=8cm and BC=15cm. The line AD bisects the angle $\displaystyle \angle$ BAC
and intersects BC at D. Given that the area of $\displaystyle \triangle$ ABD : area of $\displaystyle \triangle$ ADC =2:3,
• Aug 5th 2012, 06:45 PM
yamika
Re: Find the length of the angle bisector
Attachment 24432

from area ratio, tri ABD/tri ACD = 2/3
BD/(15-BD) =2/3
BY solving this BD = 6

From the Sine Rule for ABD triangle 6/SinA = AD/SinB

From the Sine Rule for ABD triangle 6/Sin(A)=8/Sin(A+180-(2A+B))
3/SIn(A)= 4/Sin(180-(A+B))
3/Sin(A)= 4/Sin(A+B)
4Sin(A) = 3Sin(A+B)
4Sin(A) =3[Sin(A)Cos(B) + Cos(A)Sin(B)]
Dividing by SinA , 4=3(CosB +CosASinB/SinA)
Applying from eqn 1; 4 = 3Cos(B) +3Cos(A)*AD/6
8= 6Cos(B) + AD*Cos(A) -----------Eqn 2

From the Sine Rule for triangle ACD 9/Sin(A) = AD/Sin(180-(2A+B))
9*[Sin(2A)Cos(B) +Cos(2A)Sin(B)] = AD * Sin(A)
because Sin(2A) = 2Sin(A)Cos(A) and Cos(2A) = Cos^2(A)-Sin^2(A)
Dividing both sides by Sin(A);
From eqn 2

Applying the cosine Rule for triangle ABD;
From eqn 3
• Aug 6th 2012, 05:52 AM
Soroban
Re: Find the length of the angle bisector
Hello, shiny718!

Quote:

$\displaystyle ABC\text{ is a triangle with }AB=8\text{ cm and }BC=15\text{ cm.}$

$\displaystyle \text{The line }AD\text{ bisects }\angle BAC\text{ and intersects }BC\text{ at }D.$

$\displaystyle \text{Given that (area of }\Delta ABD) : \text{(area of }\Delta ADC) \;=\;2:3,\,\text{find the length of }AD.$

Code:

                              A                               *                           *  * *               12    *    *  *                   *        *x    * 8               *          *      *           *            θ * π-θ    *     C * - - - - - - - - * - - - - - * B       :        9        D    6    :
$\displaystyle \text{Since the areas of }\Delta ABD\text{ and }\Delta ADC\text{ are in the ratio }2\!:\!3\text{ and they have the same height,}$
. . $\displaystyle \text{their bases are in the ratio }2\!:\!3.\:\text{ That is: }\:BD\!:\!DC \,=\,2\!:\!3$
$\displaystyle \text{Since }BC = 15,\,\text{ then: }\:BD = 6,\;DC = 9.$

$\displaystyle \text{Theorem: an angle bisector divides the opposite side in proportion to the other two sides.}$
. . $\displaystyle \text{Hence: }\:\tfrac{AC}{8} \,=\,\tfrac{9}{6}\quad\Rightarrow\quad AC = 12$

$\displaystyle \text{Let }x\text{ = length of the angle bisector, }AD.$
$\displaystyle \text{Let }\theta \,=\,\angle ADC \quad\Rightarrow\quad \pi-\theta \,=\,\angle ADB$

$\displaystyle \text{In }\Delta ADC,\text{ Law of Cosines: }\:\cos\theta \:=\:\frac{x^2+9^2-12^2}{2(9)(x)} \:=\:\frac{x^2-63}{18x}\;\;{\color{blue}[1]}$

$\displaystyle \text{In }\Delta ADB,\text{ Law of Cosines: }\:\cos(\pi-\theta) \:=\:\frac{x^2 + 6^2 - 8^2}{2(6)(x)} \:=\:\frac{x^2 - 28}{12x}$
. . $\displaystyle \text{This becomes: }\:-\cos\theta \:=\:\frac{x^2 - 28}{12x} \quad\Rightarrow\quad \cos\theta \:=\:\frac{28-x^2}{12x}\;\;{\color{blue}[2]}$

$\displaystyle \text{Equate }{\color{blue}[1]}\text{ and }{\color{blue}[2]}:\;\frac{x^2-63}{18x} \:=\:\frac{28-x^2}{12x}$

$\displaystyle \text{Multiply by }36x\!:\;2x^2 - 126 \:=\:84 - 3x^2 \quad\Rightarrow\quad 5x^2 \:=\:210$

. . . $\displaystyle x^2 \:=\:42 \quad\Rightarrow\quad x \:=\:\sqrt{42}$
• Aug 6th 2012, 08:57 AM
richard1234
Re: Find the length of the angle bisector
A simpler solution: We know that BD = 6 and DC = 9. By an angle bisector theorem,

$\displaystyle \frac{AC}{9} = \frac{8}{6} \Rightarrow AC = 12$.

Now we apply Stewart's theorem on triangle ABC:

$\displaystyle 8^2(9) + 12^2(6) = 15(AD^2 + 6*9)$

Solving, we get $\displaystyle AD = \sqrt{42}$.