We are given a triangle $ABC$ with $AB = 6$, $AC = 3$, and $\angle BAC = 120^\circ$. $AD$ is an angle bisector of $\angle BAC$. We are asked to find the lengths of $AD$, $AE$, and the inradii $r_1$ and $r_2$ of triangles $ADC$ and $ADE$, respectively, as well as the angle $\angle DAE$. We are given that $BE = 3\sqrt{7}$ and $CD = \sqrt{7}$.

GeometryTriangleAngle BisectorTrigonometryArea CalculationInradius
2025/6/10

1. Problem Description

We are given a triangle ABCABC with AB=6AB = 6, AC=3AC = 3, and BAC=120\angle BAC = 120^\circ. ADAD is an angle bisector of BAC\angle BAC. We are asked to find the lengths of ADAD, AEAE, and the inradii r1r_1 and r2r_2 of triangles ADCADC and ADEADE, respectively, as well as the angle DAE\angle DAE. We are given that BE=37BE = 3\sqrt{7} and CD=7CD = \sqrt{7}.

2. Solution Steps

First, we find ADAD using the area relation ABD+ACD=ABC\triangle ABD + \triangle ACD = \triangle ABC.
The area of ABC\triangle ABC is
12×AB×AC×sin(120)=12×6×3×32=932\frac{1}{2} \times AB \times AC \times \sin(120^\circ) = \frac{1}{2} \times 6 \times 3 \times \frac{\sqrt{3}}{2} = \frac{9\sqrt{3}}{2}.
The area of ABD\triangle ABD is 12×AB×AD×sin(60)=12×6×AD×32=332AD\frac{1}{2} \times AB \times AD \times \sin(60^\circ) = \frac{1}{2} \times 6 \times AD \times \frac{\sqrt{3}}{2} = \frac{3\sqrt{3}}{2} AD.
The area of ACD\triangle ACD is 12×AC×AD×sin(60)=12×3×AD×32=334AD\frac{1}{2} \times AC \times AD \times \sin(60^\circ) = \frac{1}{2} \times 3 \times AD \times \frac{\sqrt{3}}{2} = \frac{3\sqrt{3}}{4} AD.
Then 332AD+334AD=932\frac{3\sqrt{3}}{2} AD + \frac{3\sqrt{3}}{4} AD = \frac{9\sqrt{3}}{2}.
63+334AD=932\frac{6\sqrt{3} + 3\sqrt{3}}{4} AD = \frac{9\sqrt{3}}{2}.
934AD=932\frac{9\sqrt{3}}{4} AD = \frac{9\sqrt{3}}{2}.
AD=2AD = 2.
So, I=2I=2.
Next, we find DAE\angle DAE. Since ADAD is an angle bisector, BAD=CAD=60\angle BAD = \angle CAD = 60^\circ. We have CD=7CD = \sqrt{7}, so CE=BEBC=37(27+7)=0CE = BE - BC = 3\sqrt{7} - (2\sqrt{7} + \sqrt{7}) = 0, which seems incorrect.
Since BC=BD+DC=27+7=37BC = BD+DC=2\sqrt{7}+\sqrt{7}=3\sqrt{7} and BE=37BE=3\sqrt{7}, this means E=CE=C.
Since E=CE=C, AE=AC=3AE = AC = 3 and DAE=0\angle DAE=0^\circ.
Therefore, DAE=0\angle DAE = 0^\circ and AE=3AE=3. Thus JK=0JK = 0, L=3L = 3, M=1M=1.
To find r1r_1, we use the formula r1=2×AreaPerimeterr_1 = \frac{2 \times Area}{Perimeter} for ADC\triangle ADC.
The area of ADC\triangle ADC is 334AD=334×2=332\frac{3\sqrt{3}}{4}AD = \frac{3\sqrt{3}}{4} \times 2 = \frac{3\sqrt{3}}{2}.
The perimeter of ADC\triangle ADC is 3+7+2=5+73 + \sqrt{7} + 2 = 5 + \sqrt{7}.
r1=2×3323+7+2=335+7=33(57)(5+7)(57)=153321257=15332118=53216r_1 = \frac{2 \times \frac{3\sqrt{3}}{2}}{3 + \sqrt{7} + 2} = \frac{3\sqrt{3}}{5 + \sqrt{7}} = \frac{3\sqrt{3}(5 - \sqrt{7})}{(5 + \sqrt{7})(5 - \sqrt{7})} = \frac{15\sqrt{3} - 3\sqrt{21}}{25 - 7} = \frac{15\sqrt{3} - 3\sqrt{21}}{18} = \frac{5\sqrt{3} - \sqrt{21}}{6}.
Thus N=5N=5, O=3O=3, PQ=21PQ=21, R=6R=6.
Since E=CE=C, then ADEADE does not exist, which implies r2r_2 doesn't exist.
If ECE \ne C, since DAE=0\angle DAE = 0^\circ, ADAD and AEAE are on the same line, i.e., ADAD is an extension of AEAE. But ADAD bisects BAC\angle BAC. Since AC=3AC=3 and AE=3AE = 3, then C=EC = E.
Since the problem is not well-defined (E=CE=C), we cannot proceed further.
Final Answer:

1. Problem Description

We are given a triangle ABCABC with AB=6AB = 6, AC=3AC = 3, and BAC=120\angle BAC = 120^\circ. ADAD is an angle bisector of BAC\angle BAC. BE=37BE = 3\sqrt{7} and CD=7CD = \sqrt{7}.
The task is to find the lengths of ADAD, AEAE, and the inradii r1r_1 and r2r_2 of triangles ADCADC and ADEADE, respectively, and the angle DAE\angle DAE.

2. Solution Steps

AD=2AD = 2
Since E=CE=C, AE=3AE = 3 and DAE=0\angle DAE = 0^\circ
r1=53216r_1 = \frac{5\sqrt{3} - \sqrt{21}}{6}
Since E=CE=C, triangle ADE does not exist. Thus, r2r_2 cannot be calculated.

3. Final Answer

I=2I=2
JK=0JK=0
L=3L=3
M=1M=1
N=5N=5
O=3O=3
PQ=21PQ=21
R=6R=6
The values for SS, TT, UU, VV, and WW cannot be determined as E=CE=C.

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