Right Triangle Lengths

Geometry Level 1

If C D = 3 , CD=\sqrt{3}, P Q = 2 , PQ=2, and P D Q = 3 0 , \angle PDQ=30^\circ, find C P . CP.


The answer is 1.

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13 solutions

Let C D P = θ \angle CDP = \theta and C P = x CP = x .

Then tan ( θ ) = x 3 \tan(\theta) = \dfrac{x}{\sqrt{3}} and tan ( θ + 30 ) = x + 2 3 \tan(\theta + 30) = \dfrac{x + 2}{\sqrt{3}} .

Now tan ( θ + 30 ) = tan ( θ ) + tan ( 30 ) 1 tan ( θ ) tan ( 30 ) \tan(\theta + 30) = \dfrac{\tan(\theta) + \tan(30)}{1 - \tan(\theta)*\tan(30)} , so

x + 2 3 = x 3 + 1 3 1 ( x 3 ) ( 1 3 ) x + 2 = x + 1 1 x 3 \dfrac{x + 2}{\sqrt{3}} = \dfrac{\frac{x}{\sqrt{3}} + \frac{1}{\sqrt{3}}}{1 - (\frac{x}{\sqrt{3}})*(\frac{1}{\sqrt{3}})} \Longrightarrow x + 2 = \dfrac{x + 1}{1 - \frac{x}{3}}

( x + 2 ) ( 3 x ) = 3 ( x + 1 ) x 2 + 2 x 3 = 0 ( x + 3 ) ( x 1 ) = 0 \Longrightarrow (x + 2)(3 - x) = 3*(x + 1) \Longrightarrow x^{2} + 2x - 3 = 0 \Longrightarrow (x + 3)(x - 1) = 0 .

Now since we require that x > 0 x \gt 0 we conclude that x = 1 x = \boxed{1} .

why do you conclude it that it is 1

Anuj Khandare - 6 years, 6 months ago

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from that equation we get 2 roots 1 & -3 distance can not be -3 so the answer is 1...

Akash Pawadashetra - 6 years, 6 months ago

Because 1 and and -3 is the answer. And in measurements, we must always use a number greater than 0.

Cabanting Perez Francis - 6 years, 6 months ago

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Am i Correct? @anuj?

Cabanting Perez Francis - 6 years, 6 months ago

Cause length can be only positive. . not negative.. SEE....if take take eq. (x+3), then x will be -3 , agree? and if we take (x-1), x will be 1 and 1 is positive . Hope it helped :D

Harsh Kumar - 6 years, 5 months ago

it is very good!!

Cesar Molito - 6 years, 6 months ago

What exactly happened to the root3 in the equation there? Literally disappeared. Went from a fraction that simplifies into (x+1)/([1/root3]-x) to suddenly x+2.

Andres Antonatos - 5 years, 2 months ago

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Sorry, I skipped a few steps for sake of expediency. Expanding on the steps, we have

x + 2 3 = x 3 + 1 3 1 ( x 3 ) ( 1 3 ) x + 2 = 3 x + 1 3 1 x 3 x + 2 = x + 1 3 x 3 \dfrac{x + 2}{\sqrt{3}} = \dfrac{\frac{x}{\sqrt{3}} + \frac{1}{\sqrt{3}}}{1 - (\frac{x}{\sqrt{3}})*(\frac{1}{\sqrt{3}})} \Longrightarrow x + 2 = \dfrac{\sqrt{3}*\frac{x + 1}{\sqrt{3}}}{1 - \frac{x}{3}} \Longrightarrow x + 2 = \dfrac{x + 1}{\frac{3 - x}{3}}

( x + 2 ) ( 3 x ) = 3 ( x + 1 ) x 2 + 2 x 3 = 0 ( x + 3 ) ( x 1 ) = 0. \Longrightarrow (x + 2)(3 - x) = 3*(x + 1) \Longrightarrow x^{2} + 2x - 3 = 0 \Longrightarrow (x + 3)(x - 1) = 0.

Brian Charlesworth - 5 years, 2 months ago
Renah Bernat
Nov 16, 2014

Let B = CDP and X = CP:

Tan B = X/sqrt(3) Tan (30 + B) = (2 + X)/sqrt(3) By trigo identities; Tan (M +- N) = (Tan M +- Tan N)/(1 -+ Tan M Tan N) Tan 30 = 1/sqrt(3) = sqrt(3)/3 So, (Tan 30 + Tan B)/(1 - Tan 30 Tan B) = (2 + X)/sqrt(3) Subs Tan B = X/sqrt 3 Solving the equation will give X = 1

Giuliano Bianco
Apr 10, 2016

Consider triangle PDQ. By the law of cosines:

D P 2 + D Q 2 2 cos 30 ° D P P Q = 4 DP^2+DQ^2-2 \cos 30° DP \cdot PQ = 4

By Pithagoras we have:

D P 2 = C P 2 + 3 DP^2= CP^2 + 3

and

D Q 2 = ( C P + 2 ) 2 + 3 DQ^2 = (CP + 2)^2+3

As for the product of DQ and DP, let us consider the area A of triangle DPQ:

A 2 = 2 3 = D Q D P sin 30 ° \dfrac{A}{2} = 2 \sqrt{3} = DQ \cdot DP \cdot \sin 30°

hence

D P D Q = 4 3 DP \cdot DQ = 4 \sqrt{3}

Let us substitute in the first equation and solve by CP:

C P 2 + 3 + C P 2 + 2 C P + 4 + 3 2 4 3 3 2 = 4 CP^2+3+CP^2+2CP+4+3 - 2 \cdot 4\sqrt{3} \cdot \dfrac{\sqrt{3}}{2} = 4

2 C P 2 + 2 C P 6 = 0 \iff 2CP^2+2CP -6 = 0

C P 2 + C P 3 = ( C P + 3 ) ( C P 1 ) = 0 \iff CP^2+CP-3 = (CP+3) (CP-1) =0

and since CP cannot be -3, it must be 1.

Anna Anant
Nov 16, 2014

Let B = CDP and X = CP:

Tan B = X/sqrt(3) Tan (30 + B) = (2 + X)/sqrt(3) By trigo identities; Tan (M +- N) = (Tan M +- Tan N)/(1 -+ Tan M Tan N) Tan 30 = 1/sqrt(3) = sqrt(3)/3 So, (Tan 30 + Tan B)/(1 - Tan 30 Tan B) = (2 + X)/sqrt(3) Subs Tan B = X/sqrt 3 Solving the equation will give X = 1

Amed Lolo
Jan 25, 2016

area of∆ dcq=1\2×(2+R)×√3=area∆ cpd+area ∆pqd=1\2×R×√3+1\2×M×N×sin30,R=cp,M=Dp,N=DQ,OK ,,1\2×(2+R)√3=1\2×R×√3+1\2×M×N×1\2,,,so 2√3+R√3=R√3+M×N×1\2,M×N=4√3,,,,,,,,,,,,,,,, cos30=M^2+N^2-4\2M.N ,√3×M.N=M^2+N^2-4. M×N=4√3 ,so 4√3×√3=M^2+48\M^2-4. M^2+48\M^2=16,multiple expression by M^2. M^4-16M^2+48=0.(m^2-12).(m^2-4)=0 so m^2=12 Or 4 , m^2. N^2=48 so ,N^2=48\12=4 . N=2 rejected N>PQ>2.,,,so M^2=2 , . R^2=2^2-√3^2=1.R=1=CP###########

Ranjitha Gowda
Jan 2, 2015

Length of PQ=length of PD By using Pythagora's thm PD^{2}=DC^{2}+CP^{2} thus to get the length of CP, CP=\sqrt{2^2-3} so CP=1

Yawar Aftab
Nov 16, 2014

triangle DCP is a special triangle... a 30-60-90 triangle whose sides are always x-x*3^1/2-2x... once you see that the question pretty much solves itself.

Where do get that the triangle is 30-60-90? I assumed it to get the answer, but nowhere does it say that segment D P DP bisects angle C D Q CDQ , that D P = P Q DP=PQ , or that D Q = 2 3 DQ=2 * \sqrt{3}

Stephen Hanna - 6 years, 6 months ago

put, CP=X, area of CDQ(1/2 x root 3 x X+2) = area CDP(1/2 x root 3 x X)+ area DPQ ( 1/2 x sin 30 x DP x DQ)...........(1)

here, DP = root (3+x^2) and DQ = root(3+ (X+2)^2) solve eqn (1), the possible value of it comes 1, hence CP=1

Shyam Bihari
Nov 16, 2014

triangle DPQ having base-2 m and perpendicular height-root3 .then
tan 30=(2+x)/root3; 1/root3=(2+x)/root3; 2+x=1. hence x=-1(where x is length of CP) solength of CP=1

Mahmoud Elhossany
Nov 16, 2014

Triangle DCQ is a 30-60-90 triangle since angle CDQ = 60 and angle C = 90. Since angle Q = 30, triangle PDQ is isosceles which tells us that DP = 2m. Now use the Pythagorean theorem on triangle CDP to find CP: CD^2+CP^2=DP^2 3 + CP^2 = 4

which tells us CP = 1

Gino Pagano
Nov 11, 2014

Triangle DCQ is a 30-60-90 triangle since angle CDQ = 60 and angle C = 90. Since angle Q = 30, triangle PDQ is isosceles which tells us that DP = 2m. Now use the Pythagorean theorem on triangle CDP to find CP:

3 + CP^2 = 4

which tells us CP = 1

How did you figure out that triangle dcq is a 30-60-90 triangle? ??

Istiak Reza - 6 years, 6 months ago
Sanjeet Raria
Sep 18, 2014

Of course there's a rigorous method to find the length but here's what i did- i randomly chose the value of angle CPD, first 30 but that means CDQ is 90 that's absurd. Then i put CPD=60, that means CQD=30=>DP=PQ=2 & CP=1 & i checked this combination using Pythagoras and it worked!! Pretty much saved my time.

Michele Caselli
Sep 18, 2014

I made system with , Pitagora' s theorem , Stweart's theorem and law of cosine, solve it to find DQ , CP and DP

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