Solve it if you can #2

Geometry Level 4

In the triangle ABC, if AB=6, AC=3 and the angle (B)=30 and D is the base of the bisector of (A), what is the distance between D to AB?


The answer is 1.73.

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3 solutions

Armita Kazemi
Aug 20, 2015

According to a theorem: BD/CD=sin⁡(BAD) ̂ /sin⁡(DAC) ̂ ×AB/AC we replace AB=6 , AC=3 in the formula (we knew that angles BAD and DAC were equal so sin BAD=sin DAC) First=> 2CD=BD then we use cosines theorem: AC^2 = AB^2 + BC^2 -2 AB BC Cos ABC by replacing values of AB and BC and Cos ABC and solving the outcome quadratic equation, we will understand that: Second=> BC=3 3^(1/2) by the First and Second equations we can see that BD=2 (3^(1/2)). and its obvious that distance between D and side AB is equal to BD (1\2) means the distance is = 3^(1\2)

D r o p D P B C . W h a t w e n e e d i s P D . A 30 60 90 r t d Δ i s s h o w n i n t h e s k e t c h . From this we can see that ABC, ABD, APD are all 30-60-90 Δ s i n Δ A C D , A D = 2 3 . i n Δ A P D , P D = 2 3 1 2 = 1.73 Drop~DP\perp ~BC. ~What~ we~ need ~is~\color{#D61F06}{ PD}.\\ A~ 30-60-90 ~rt\angle d~ \Delta~is~shown~in~the~sketch.\\ \text{From this we can see that ABC, ABD, APD are all 30-60-90 } \Delta s\\ \therefore~in~\Delta~ACD,~~AD=2\sqrt3.\\ \therefore~in~\Delta~APD,~~\color{#D61F06}{ PD}=2\sqrt3*\dfrac 1 2=1.73

Why AD = 2√3?

A Former Brilliant Member - 5 years, 10 months ago

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Δ A C D i s 30 60 90. H y p o t . A D = 2 3 . A C = 2 3 3 = 2 3 . \Delta~ACD~is~30-60-90.\\ Hypot. AD=\dfrac 2 {\sqrt3}. AC=\dfrac 2 {\sqrt3}*3=2\sqrt3.

Niranjan Khanderia - 5 years, 10 months ago

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I think BC= √6^2+3^2=√45=3√5. Because sin(30°)=0.5, AD=2 DC. Obviously ADC ≌ BPD, so BD=2 DC. Then DC=√5.

A Former Brilliant Member - 5 years, 10 months ago

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@A Former Brilliant Member B C = 6 2 3 2 = 3 3 . BC=\sqrt{6^2 \color{#D61F06}{ \Large -}3^2}=3\sqrt3.

Niranjan Khanderia - 5 years, 10 months ago

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@Niranjan Khanderia Oh lord, I get it now! I made a big mistake...

A Former Brilliant Member - 5 years, 10 months ago

I think BD = 2DC. Obviously BC = √(6^2+3^2)=3√5. So DC = √5

A Former Brilliant Member - 5 years, 10 months ago

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Why BD=2 * DC?

Niranjan Khanderia - 5 years, 10 months ago

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Because sin(30°)=0.5, AD=2 DC. Obviously ADC ≌ BPD, so BD=2 DC.

A Former Brilliant Member - 5 years, 10 months ago

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@A Former Brilliant Member Sorry, you are correct. You have slipped in finding BC as I have commented above.

Niranjan Khanderia - 5 years, 10 months ago

B C = 6 2 3 2 = 3 3 . BC=\sqrt{6^2 \color{#D61F06}{ \Large -}3^2}=3\sqrt3.

Niranjan Khanderia - 5 years, 10 months ago

According to Sin theory, we have: A C s i n ( B ) = A B s i n ( C ) C = 9 0 0 \frac{|AC|}{sin(B)}=\frac{|AB|}{sin(C)} \rightarrow C=90^{0} B C = A B 2 A C 2 BC=\sqrt{|AB|^{2}-|AC|^{2}} d D A B = [ A B C ] A B + A C = 1.73 d_{D|AB}=\frac{[ABC]}{|AB|+|AC|}=\boxed{1.73}

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