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Straight Lines and Pair of Straight Lines question

2022 · 28 Jun · Shift 2 · Q33
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  5. /2022 · 28 Jun · Shift 2 · Q33

Straight Lines and Pair of Straight Lines question

2022 · 28 Jun · Shift 2 · Q33

JEE MainMathematicsStraight Lines and Pair of Straight LinesMCQ+4 / −1
Let a triangle be bounded by the lines L1 : 2x + 5y = 10; L2 : −-− 4x + 3y = 12 and the line L3, which passes through the point P(2, 3), intersects L2 at A and L1 at B. If the point P divides the line-segment AB, internally in the ratio 1 : 3, then the area of the triangle is equal to :
  1. A
    11013{{110} \over {13}}13110​
  2. B
    13213{{132} \over {13}}13132​
  3. C
    14213{{142} \over {13}}13142​
  4. D
    15113{{151} \over {13}}13151​
View written solutionFree

Correct answer: B

  1. Given lines

L1:2x+5y=10,L2:−4x+3y=12L_1: 2x+5y=10, \qquad L_2:-4x+3y=12L1​:2x+5y=10,L2​:−4x+3y=12

A third line L3L_3L3​ passes through P(2,3)P(2,3)P(2,3), meets L2L_2L2​ at AAA and L1L_1L1​ at BBB.

Also, PPP divides ABABAB internally in the ratio 1:31:31:3.


  1. Use section formula

Since PPP divides ABABAB internally in the ratio 1:31:31:3, we interpret

AP:PB=1:3.AP:PB=1:3.AP:PB=1:3.

If A(x1,y1)A(x_1,y_1)A(x1​,y1​) and B(x2,y2)B(x_2,y_2)B(x2​,y2​), then by section formula,

P=(1⋅x2+3⋅x11+3,  1⋅y2+3⋅y11+3).P=\left(\frac{1\cdot x_2+3\cdot x_1}{1+3},\;\frac{1\cdot y_2+3\cdot y_1}{1+3}\right).P=(1+31⋅x2​+3⋅x1​​,1+31⋅y2​+3⋅y1​​).

So,

4P=B+3A.4P = B+3A.4P=B+3A.

Since P=(2,3)P=(2,3)P=(2,3),

4P=(8,12).4P=(8,12).4P=(8,12).

Hence,

B+3A=(8,12).B+3A=(8,12).B+3A=(8,12).

Let

A=(x1,y1),B=(x2,y2).A=(x_1,y_1), \quad B=(x_2,y_2).A=(x1​,y1​),B=(x2​,y2​).

Because AAA lies on L2L_2L2​ and BBB lies on L1L_1L1​,

−4x1+3y1=12...(1)-4x_1+3y_1=12 \quad ...(1)−4x1​+3y1​=12...(1) 2x2+5y2=10...(2)2x_2+5y_2=10 \quad ...(2)2x2​+5y2​=10...(2)

and from B+3A=(8,12)B+3A=(8,12)B+3A=(8,12),

x2+3x1=8...(3)x_2+3x_1=8 \quad ...(3)x2​+3x1​=8...(3) y2+3y1=12...(4)y_2+3y_1=12 \quad ...(4)y2​+3y1​=12...(4)


  1. Solve for AAA and BBB

From (3) and (4),

x2=8−3x1,y2=12−3y1.x_2=8-3x_1, \qquad y_2=12-3y_1.x2​=8−3x1​,y2​=12−3y1​.

Substitute into (2):

2(8−3x1)+5(12−3y1)=102(8-3x_1)+5(12-3y_1)=102(8−3x1​)+5(12−3y1​)=10 16−6x1+60−15y1=1016-6x_1+60-15y_1=1016−6x1​+60−15y1​=10 6x1+15y1=666x_1+15y_1=666x1​+15y1​=66 2x1+5y1=22...(5)2x_1+5y_1=22 \quad ...(5)2x1​+5y1​=22...(5)

Now solve (1) and (5):

−4x1+3y1=12-4x_1+3y_1=12−4x1​+3y1​=12 2x1+5y1=222x_1+5y_1=222x1​+5y1​=22

Multiply the second by 222:

4x1+10y1=444x_1+10y_1=444x1​+10y1​=44

Add with the first:

13y1=56⇒y1=5613.13y_1=56 \Rightarrow y_1=\frac{56}{13}.13y1​=56⇒y1​=1356​.

Then from (5):

2x1+5⋅5613=222x_1+5\cdot \frac{56}{13}=222x1​+5⋅1356​=22 2x1=22−28013=286−28013=6132x_1=22-\frac{280}{13}=\frac{286-280}{13}=\frac{6}{13}2x1​=22−13280​=13286−280​=136​ x1=313.x_1=\frac{3}{13}.x1​=133​.

Therefore,

A=(313,5613).A=\left(\frac{3}{13},\frac{56}{13}\right).A=(133​,1356​).

Now,

x2=8−3⋅313=104−913=9513,x_2=8-3\cdot \frac{3}{13}=\frac{104-9}{13}=\frac{95}{13},x2​=8−3⋅133​=13104−9​=1395​, y2=12−3⋅5613=156−16813=−1213.y_2=12-3\cdot \frac{56}{13}=\frac{156-168}{13}=-\frac{12}{13}.y2​=12−3⋅1356​=13156−168​=−1312​.

Hence,

B=(9513,−1213).B=\left(\frac{95}{13},-\frac{12}{13}\right).B=(1395​,−1312​).


  1. Find the third vertex of the triangle

The triangle is bounded by L1,L2,L3L_1, L_2, L_3L1​,L2​,L3​.

Its vertices are:

  • A=L2∩L3A = L_2 \cap L_3A=L2​∩L3​
  • B=L1∩L3B = L_1 \cap L_3B=L1​∩L3​
  • C=L1∩L2C = L_1 \cap L_2C=L1​∩L2​

Find CCC by solving:

2x+5y=102x+5y=102x+5y=10 −4x+3y=12-4x+3y=12−4x+3y=12

Multiply first by 222:

4x+10y=204x+10y=204x+10y=20

Add with second:

13y=32⇒y=3213.13y=32 \Rightarrow y=\frac{32}{13}.13y=32⇒y=1332​.

Then

2x+5⋅3213=102x+5\cdot \frac{32}{13}=102x+5⋅1332​=10 2x=10−16013=130−16013=−30132x=10-\frac{160}{13}=\frac{130-160}{13}=-\frac{30}{13}2x=10−13160​=13130−160​=−1330​ x=−1513.x=-\frac{15}{13}.x=−1315​.

So,

C=(−1513,3213).C=\left(-\frac{15}{13},\frac{32}{13}\right).C=(−1315​,1332​).


  1. Compute area of triangle ABCABCABC

Use determinant formula:

Area=12∣x1(y2−y3)+x2(y3−y1)+x3(y1−y2)∣.\text{Area}=\frac12\left|x_1(y_2-y_3)+x_2(y_3-y_1)+x_3(y_1-y_2)\right|.Area=21​∣x1​(y2​−y3​)+x2​(y3​−y1​)+x3​(y1​−y2​)∣.

Take

A(313,5613),B(9513,−1213),C(−1513,3213).A\left(\frac{3}{13},\frac{56}{13}\right), \quad B\left(\frac{95}{13},-\frac{12}{13}\right), \quad C\left(-\frac{15}{13},\frac{32}{13}\right).A(133​,1356​),B(1395​,−1312​),C(−1315​,1332​).

Then

y2−y3=−1213−3213=−4413y_2-y_3=-\frac{12}{13}-\frac{32}{13}=-\frac{44}{13}y2​−y3​=−1312​−1332​=−1344​ y3−y1=3213−5613=−2413y_3-y_1=\frac{32}{13}-\frac{56}{13}=-\frac{24}{13}y3​−y1​=1332​−1356​=−1324​ y1−y2=5613−(−1213)=6813y_1-y_2=\frac{56}{13}-\left(-\frac{12}{13}\right)=\frac{68}{13}y1​−y2​=1356​−(−1312​)=1368​

So,

Area=12∣313⋅(−4413)+9513⋅(−2413)+(−1513)⋅6813∣\text{Area}=\frac12\left|\frac{3}{13}\cdot \left(-\frac{44}{13}\right)+\frac{95}{13}\cdot \left(-\frac{24}{13}\right)+\left(-\frac{15}{13}\right)\cdot \frac{68}{13}\right|Area=21​​133​⋅(−1344​)+1395​⋅(−1324​)+(−1315​)⋅1368​​

=12∣−132−2280−1020169∣=\frac12\left|\frac{-132-2280-1020}{169}\right|=21​​169−132−2280−1020​​ =12⋅3432169=\frac12\cdot \frac{3432}{169}=21​⋅1693432​ =1716169=\frac{1716}{169}=1691716​

Since 169=132169=13^2169=132 and 1716=132⋅131716=132\cdot 131716=132⋅13,

Area=13213.\text{Area}=\frac{132}{13}.Area=13132​.


  1. Check with options

13213\boxed{\frac{132}{13}}13132​​

So the correct option is B.


  1. Comparison with stored answer

Stored correct answer: B

Derived answer: B

They agree.

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