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Straight Lines and Pair of Straight Lines question

2021 · 16 Mar · Shift 2 · Q38
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  5. /2021 · 16 Mar · Shift 2 · Q38

Straight Lines and Pair of Straight Lines question

2021 · 16 Mar · Shift 2 · Q38

JEE MainMathematicsStraight Lines and Pair of Straight LinesMCQ+4 / −1
Let A(−-− 1, 1), B(3, 4) and C(2, 0) be given three points. A line y = mx, m > 0, intersects lines AC and BC at point P and Q respectively. Let A1 and A2 be the areas of Δ\DeltaΔ ABC and Δ\DeltaΔ PQC respectively, such that A1 = 3A2, then the value of m is equal to :
  1. A
    1
  2. B
    3
  3. C
    2
  4. D
    415{4 \over {15}}154​
View written solutionFree

Correct answer: A

  1. Given points

A(−1,1),B(3,4),C(2,0)A(-1,1),\quad B(3,4),\quad C(2,0)A(−1,1),B(3,4),C(2,0)

A line through the origin is

y=mx,m>0y=mx, \quad m>0y=mx,m>0

It intersects line ACACAC at PPP and line BCBCBC at QQQ.

We are given

A1=[△ABC],A2=[△PQC],A1=3A2A_1=[\triangle ABC], \quad A_2=[\triangle PQC], \quad A_1=3A_2A1​=[△ABC],A2​=[△PQC],A1​=3A2​

So,

[△PQC]=13[△ABC][\triangle PQC]=\frac13[\triangle ABC][△PQC]=31​[△ABC]


  1. Equation of line ACACAC

Slope of ACACAC:

0−12−(−1)=−13\frac{0-1}{2-(-1)}=-\frac132−(−1)0−1​=−31​

So equation through A(−1,1)A(-1,1)A(−1,1):

y−1=−13(x+1)y-1=-\frac13(x+1)y−1=−31​(x+1)

3y−3=−x−13y-3=-x-13y−3=−x−1

x+3y−2=0x+3y-2=0x+3y−2=0

Hence,

AC:x+3y−2=0AC: x+3y-2=0AC:x+3y−2=0


  1. Equation of line BCBCBC

Slope of BCBCBC:

0−42−3=4\frac{0-4}{2-3}=42−30−4​=4

Equation through C(2,0)C(2,0)C(2,0):

y=4(x−2)y=4(x-2)y=4(x−2)

y=4x−8y=4x-8y=4x−8

Hence,

BC:4x−y−8=0BC: 4x-y-8=0BC:4x−y−8=0


  1. Coordinates of PPP

PPP lies on both ACACAC and y=mxy=mxy=mx.

Substitute y=mxy=mxy=mx into x+3y−2=0x+3y-2=0x+3y−2=0:

x+3mx−2=0x+3mx-2=0x+3mx−2=0

x(1+3m)=2x(1+3m)=2x(1+3m)=2

xP=21+3mx_P=\frac{2}{1+3m}xP​=1+3m2​

yP=mxP=2m1+3my_P=m x_P=\frac{2m}{1+3m}yP​=mxP​=1+3m2m​

Thus,

P(21+3m,2m1+3m)P\left(\frac{2}{1+3m},\frac{2m}{1+3m}\right)P(1+3m2​,1+3m2m​)


  1. Coordinates of QQQ

QQQ lies on both BCBCBC and y=mxy=mxy=mx.

Substitute y=mxy=mxy=mx into y=4x−8y=4x-8y=4x−8:

mx=4x−8mx=4x-8mx=4x−8

(4−m)x=8(4-m)x=8(4−m)x=8

xQ=84−mx_Q=\frac{8}{4-m}xQ​=4−m8​

yQ=mxQ=8m4−my_Q=m x_Q=\frac{8m}{4-m}yQ​=mxQ​=4−m8m​

Thus,

Q(84−m,8m4−m)Q\left(\frac{8}{4-m},\frac{8m}{4-m}\right)Q(4−m8​,4−m8m​)


  1. Use area ratio via similarity

Since PPP lies on ACACAC, QQQ lies on BCBCBC, and PQPQPQ is along the same direction as line through origin, triangle PQCPQCPQC is formed inside triangle ABCABCABC with vertex CCC and sides along CACACA and CBCBCB.

Let us parametrize points on the two sides from CCC.

For point PPP on ACACAC:

A−C=(−1−2,1−0)=(−3,1)A-C=(-1-2,1-0)=(-3,1)A−C=(−1−2,1−0)=(−3,1)

So any point on ACACAC is

C+t(A−C)=(2−3t,t)C+t(A-C)=(2-3t,t)C+t(A−C)=(2−3t,t)

Comparing with PPP:

yP=t=2m1+3my_P=t=\frac{2m}{1+3m}yP​=t=1+3m2m​

Hence,

CPCA=t=2m1+3m\frac{CP}{CA}=t=\frac{2m}{1+3m}CACP​=t=1+3m2m​

For point QQQ on BCBCBC:

B−C=(3−2,4−0)=(1,4)B-C=(3-2,4-0)=(1,4)B−C=(3−2,4−0)=(1,4)

So any point on BCBCBC is

C+s(B−C)=(2+s,4s)C+s(B-C)=(2+s,4s)C+s(B−C)=(2+s,4s)

Comparing with QQQ:

4s=8m4−m4s=\frac{8m}{4-m}4s=4−m8m​

s=2m4−ms=\frac{2m}{4-m}s=4−m2m​

Hence,

CQCB=s=2m4−m\frac{CQ}{CB}=s=\frac{2m}{4-m}CBCQ​=s=4−m2m​

Now,

[△PQC]=CPCA⋅CQCB⋅[△ABC][\triangle PQC]=\frac{CP}{CA}\cdot \frac{CQ}{CB}\cdot [\triangle ABC][△PQC]=CACP​⋅CBCQ​⋅[△ABC]

Therefore,

A2A1=2m1+3m⋅2m4−m\frac{A_2}{A_1}=\frac{2m}{1+3m}\cdot\frac{2m}{4-m}A1​A2​​=1+3m2m​⋅4−m2m​

Given A1=3A2A_1=3A_2A1​=3A2​, so

A2A1=13\frac{A_2}{A_1}=\frac13A1​A2​​=31​

Thus,

2m1+3m⋅2m4−m=13\frac{2m}{1+3m}\cdot\frac{2m}{4-m}=\frac131+3m2m​⋅4−m2m​=31​


  1. Solve for mmm

4m2(1+3m)(4−m)=13\frac{4m^2}{(1+3m)(4-m)}=\frac13(1+3m)(4−m)4m2​=31​

12m2=(1+3m)(4−m)12m^2=(1+3m)(4-m)12m2=(1+3m)(4−m)

Expand RHS:

(1+3m)(4−m)=4−m+12m−3m2=4+11m−3m2(1+3m)(4-m)=4-m+12m-3m^2=4+11m-3m^2(1+3m)(4−m)=4−m+12m−3m2=4+11m−3m2

So,

12m2=4+11m−3m212m^2=4+11m-3m^212m2=4+11m−3m2

15m2−11m−4=015m^2-11m-4=015m2−11m−4=0

Factorize:

15m2−15m+4m−4=015m^2-15m+4m-4=015m2−15m+4m−4=0

15m(m−1)+4(m−1)=015m(m-1)+4(m-1)=015m(m−1)+4(m−1)=0

(m−1)(15m+4)=0(m-1)(15m+4)=0(m−1)(15m+4)=0

So,

m=1orm=−415m=1 \quad \text{or} \quad m=-\frac{4}{15}m=1orm=−154​

Given m>0m>0m>0, hence

m=1m=1m=1


  1. Check with options

Option A: 111 ✅

So the correct answer is

1\boxed{1}1​


  1. Comparison with stored correct answer

Stored correct answer: A

Our derived answer is also Option A, i.e. m=1m=1m=1.

So the stored answer is correct.

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