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Vector Algebra question

2019 · 11 Jan · Shift 2 · Q42
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Vector Algebra question

2019 · 11 Jan · Shift 2 · Q42

JEE MainMathematicsVector AlgebraMCQ+4 / −1
Let 3i^+j^,i^+3j^\sqrt 3 \widehat i + \widehat j,\widehat i + \sqrt 3 \widehat j3​i+j​,i+3​j​ and βi^+(1−β)j^\beta \widehat i + \left( {1 - \beta } \right)\widehat jβi+(1−β)j​ respectively be the position vectors of the points A, B and C with respect to the origin O. If the distance of C from the bisector of the acute angle between OA and OB is 32{3 \over {\sqrt 2 }}2​3​, then the sum of all possible values of β\betaβ is :
  1. A
    4
  2. B
    1
  3. C
    2
  4. D
    3
View written solutionFree

Correct answer: B

  1. Write the coordinates of the points

The given position vectors are:

OA⃗=3 i^+j^,OB⃗=i^+3 j^,OC⃗=β i^+(1−β) j^\vec{OA}=\sqrt{3}\,\hat i+\hat j,\qquad \vec{OB}=\hat i+\sqrt{3}\,\hat j,\qquad \vec{OC}=\beta\,\hat i+(1-\beta)\,\hat jOA=3​i^+j^​,OB=i^+3​j^​,OC=βi^+(1−β)j^​

So,

A(3,1),B(1,3),C(β,1−β).A(\sqrt{3},1),\qquad B(1,\sqrt{3}),\qquad C(\beta,1-\beta).A(3​,1),B(1,3​),C(β,1−β).


  1. Find the bisector of the acute angle between OAOAOA and OBOBOB

The vectors OA⃗\vec{OA}OA and OB⃗\vec{OB}OB have equal magnitudes:

∣OA⃗∣=3+1=2,∣OB⃗∣=1+3=2.|\vec{OA}|=\sqrt{3+1}=2,\qquad |\vec{OB}|=\sqrt{1+3}=2.∣OA∣=3+1​=2,∣OB∣=1+3​=2.

Hence the internal angle bisector is along

OA⃗∣OA⃗∣+OB⃗∣OB⃗∣\frac{\vec{OA}}{|\vec{OA}|}+\frac{\vec{OB}}{|\vec{OB}|}∣OA∣OA​+∣OB∣OB​

Since both magnitudes are equal, this is just along

OA⃗+OB⃗=(3+1)i^+(3+1)j^.\vec{OA}+\vec{OB}=(\sqrt{3}+1)\hat i+(\sqrt{3}+1)\hat j.OA+OB=(3​+1)i^+(3​+1)j^​.

So the bisector has direction (1,1)(1,1)(1,1), hence its equation is

y=x.y=x.y=x.


  1. Use the distance of point CCC from the line y=xy=xy=x

Line y=xy=xy=x can be written as

x−y=0.x-y=0.x−y=0.

Distance of point (x1,y1)(x_1,y_1)(x1​,y1​) from line ax+by+c=0ax+by+c=0ax+by+c=0 is

d=∣ax1+by1+c∣a2+b2.d=\frac{|ax_1+by_1+c|}{\sqrt{a^2+b^2}}.d=a2+b2​∣ax1​+by1​+c∣​.

Here, for C(β,1−β)C(\beta,1-\beta)C(β,1−β),

d=∣β−(1−β)∣12+(−1)2=∣2β−1∣2.d=\frac{|\beta-(1-\beta)|}{\sqrt{1^2+(-1)^2}}=\frac{|2\beta-1|}{\sqrt{2}}.d=12+(−1)2​∣β−(1−β)∣​=2​∣2β−1∣​.

Given that this distance is

32,\frac{3}{\sqrt{2}},2​3​,

so

∣2β−1∣2=32.\frac{|2\beta-1|}{\sqrt{2}}=\frac{3}{\sqrt{2}}.2​∣2β−1∣​=2​3​.

Therefore,

∣2β−1∣=3.|2\beta-1|=3.∣2β−1∣=3.

So,

2β−1=3or2β−1=−3.2\beta-1=3 \quad \text{or} \quad 2\beta-1=-3.2β−1=3or2β−1=−3.

Thus,

β=2orβ=−1.\beta=2 \quad \text{or} \quad \beta=-1.β=2orβ=−1.


  1. Find the sum of all possible values of β\betaβ

2+(−1)=1.2+(-1)=1.2+(−1)=1.


  1. Check with options

The correct option is:

B: 1\boxed{\text{B: }1}B: 1​


  1. Comparison with stored answer

Stored correct answer = B

Our derived answer = B

So they agree.

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