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NCERT Solutions

Pair of Linear Equations in Two Variables

Madhya Pradesh Board · Class 10 · Mathematics

NCERT Solutions for Pair of Linear Equations in Two Variables — Madhya Pradesh Board Class 10 Mathematics.

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EXERCISE 3.1

1Form the pair of linear equations in the following problems, and find their solutions graphically.Show solution
Let the number of boys be xx and the number of girls be yy.

From the problem:
- Total students: x+y=10x+y=10
- Girls are 4 more than boys: y=x+4y=x+4

So the pair of linear equations is:
x+y=10,y=x+4 x+y=10,\qquad y=x+4

Substitute y=x+4y=x+4 into x+y=10x+y=10:
x+(x+4)=10 x+(x+4)=10
2x+4=10 2x+4=10
2x=6 2x=6
x=3 x=3
Then
y=x+4=3+4=7 y=x+4=3+4=7

So the graphical solution would be the point (3,7)(3,7).

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1(i)10 students of Class X took part in a Mathematics quiz. If the number of girls is 4 more than the number of boys, find the number of boys and girls who took part in the quiz.Show solution
Let the number of boys be xx and the number of girls be yy.

Then:
x+y=10 x+y=10
y=x+4 y=x+4

Substitute:
x+(x+4)=10 x+(x+4)=10
2x=6 2x=6
x=3 x=3
So,
y=3+4=7 y=3+4=7

Hence, there were 3 boys and 7 girls.

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1(ii)5 pencils and 7 pens together cost ₹ 50, whereas 7 pencils and 5 pens together cost ₹ 46. Find the cost of one pencil and that of one pen.Show solution
Let the cost of one pencil be xx and one pen be yy.

From the problem:
5x+7y=50 5x+7y=50
7x+5y=46 7x+5y=46

Eliminate yy:
Multiply the first equation by 5 and the second by 7:
25x+35y=250 25x+35y=250
49x+35y=322 49x+35y=322
Subtract:
24x=72 24x=72
x=3 x=3

Substitute in 5x+7y=505x+7y=50:
5(3)+7y=50 5(3)+7y=50
15+7y=50 15+7y=50
7y=35 7y=35
y=5 y=5

So the cost is ₹3 per pencil and ₹5 per pen.

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2On comparing the ratios a1a2\frac{a_1}{a_2}, b1b2\frac{b_1}{b_2} and c1c2\frac{c_1}{c_2}, find out whether the lines representing the following pairs of linear equations intersect at a point, are parallel or coincident:Show solution
To decide whether the lines are intersecting, parallel, or coincident, compare the ratios a1a2\frac{a_1}{a_2}, b1b2\frac{b_1}{b_2} and c1c2\frac{c_1}{c_2} for each pair.

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2(i)5x4y+8=05x - 4y + 8 = 0

7x+6y9=07x + 6y - 9 = 0
Show solution
For 5x4y+8=05x-4y+8=0 and 7x+6y9=07x+6y-9=0:
a1a2=57,b1b2=46 \frac{a_1}{a_2}=\frac{5}{7},\quad \frac{b_1}{b_2}=\frac{-4}{6}
Since
5746, \frac{5}{7}\ne \frac{-4}{6},
the lines intersect at a point.

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2(ii)9x+3y+12=09x + 3y + 12 = 0

18x+6y+24=018x + 6y + 24 = 0
Show solution
For 9x+3y+12=09x+3y+12=0 and 18x+6y+24=018x+6y+24=0:
a1a2=918=12,b1b2=36=12,c1c2=1224=12 \frac{a_1}{a_2}=\frac{9}{18}=\frac12,\quad \frac{b_1}{b_2}=\frac{3}{6}=\frac12,\quad \frac{c_1}{c_2}=\frac{12}{24}=\frac12
Since
a1a2=b1b2=c1c2, \frac{a_1}{a_2}=\frac{b_1}{b_2}=\frac{c_1}{c_2},
the lines are coincident.

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2(iii)6x3y+10=06x - 3y + 10 = 0

2xy+9=02x - y + 9 = 0
Show solution
For 6x3y+10=06x-3y+10=0 and 2xy+9=02x-y+9=0:
a1a2=62=3,b1b2=31=3,c1c2=109 \frac{a_1}{a_2}=\frac{6}{2}=3,\quad \frac{b_1}{b_2}=\frac{-3}{-1}=3,\quad \frac{c_1}{c_2}=\frac{10}{9}
Since
a1a2=b1b2c1c2, \frac{a_1}{a_2}=\frac{b_1}{b_2}\ne \frac{c_1}{c_2},
the lines are parallel.

The computed relation shows they are parallel, so if the printed options differ, the correct classification is parallel lines.

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3On comparing the ratios a1a2\frac{a_1}{a_2}, b1b2\frac{b_1}{b_2} and c1c2\frac{c_1}{c_2}, find out whether the following pair of linear equations are consistent, or inconsistent.Show solution
Use the ratio test:
- If a1a2b1b2\frac{a_1}{a_2} \ne \frac{b_1}{b_2}, the pair is consistent.
- If a1a2=b1b2c1c2\frac{a_1}{a_2} = \frac{b_1}{b_2} \ne \frac{c_1}{c_2}, the pair is inconsistent.
- If a1a2=b1b2=c1c2\frac{a_1}{a_2} = \frac{b_1}{b_2} = \frac{c_1}{c_2}, the pair is consistent with infinitely many solutions.

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3(i)3x+2y=53x + 2y = 5; 2x3y=72x - 3y = 7Show solution
For 3x+2y=53x+2y=5 and 2x3y=72x-3y=7:
a1a2=32,b1b2=23 \frac{a_1}{a_2}=\frac{3}{2},\quad \frac{b_1}{b_2}=\frac{2}{-3}
Since
3223, \frac{3}{2}\ne \frac{2}{-3},
the pair is consistent.

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3(ii)2x3y=82x - 3y = 8; 4x6y=94x - 6y = 9Show solution
For 2x3y=82x-3y=8 and 4x6y=94x-6y=9:
a1a2=24=12,b1b2=36=12,c1c2=89 \frac{a_1}{a_2}=\frac{2}{4}=\frac12,\quad \frac{b_1}{b_2}=\frac{-3}{-6}=\frac12,\quad \frac{c_1}{c_2}=\frac{8}{9}
Since
a1a2=b1b2c1c2, \frac{a_1}{a_2}=\frac{b_1}{b_2}\ne \frac{c_1}{c_2},
the pair is inconsistent.

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3(iii)32x+53y=7\frac{3}{2}x + \frac{5}{3}y = 7; 9x10y=149x - 10y = 14Show solution
For 32x+53y=7\frac{3}{2}x+\frac{5}{3}y=7 and 9x10y=149x-10y=14:

Write in standard form:
32x+53y7=0,9x10y14=0 \frac{3}{2}x+\frac{5}{3}y-7=0,\qquad 9x-10y-14=0
So,
a1a2=3/29=16,b1b2=5/310=16 \frac{a_1}{a_2}=\frac{3/2}{9}=\frac{1}{6},\quad \frac{b_1}{b_2}=\frac{5/3}{-10}=-\frac{1}{6}
Since
a1a2b1b2, \frac{a_1}{a_2}\ne \frac{b_1}{b_2},
the pair is consistent.

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3(iv)5x3y=115x - 3y = 11; 10x+6y=22-10x + 6y = -22Show solution
For 5x3y=115x-3y=11 and 10x+6y=22-10x+6y=-22:
a1a2=510=12,b1b2=36=12,c1c2=1122=12 \frac{a_1}{a_2}=\frac{5}{-10}=-\frac12,\quad \frac{b_1}{b_2}=\frac{-3}{6}=-\frac12,\quad \frac{c_1}{c_2}=\frac{11}{-22}=-\frac12
Since all three ratios are equal, the pair is dependent and consistent.

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3(v)43x+2y=8\frac{4}{3}x + 2y = 8; 2x+3y=122x + 3y = 12Show solution
For 43x+2y=8\frac{4}{3}x+2y=8 and 2x+3y=122x+3y=12:
a1a2=4/32=23,b1b2=23 \frac{a_1}{a_2}=\frac{4/3}{2}=\frac{2}{3},\quad \frac{b_1}{b_2}=\frac{2}{3}
Also,
c1c2=812=23 \frac{c_1}{c_2}=\frac{8}{12}=\frac{2}{3}
Thus,
a1a2=b1b2=c1c2, \frac{a_1}{a_2}=\frac{b_1}{b_2}=\frac{c_1}{c_2},
so the pair is dependent and consistent.

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4Which of the following pairs of linear equations are consistent/inconsistent? If consistent, obtain the solution graphically:Show solution
Check each pair using the ratio criterion:
- If a1a2b1b2\frac{a_1}{a_2} \ne \frac{b_1}{b_2}, the pair is consistent.
- If a1a2=b1b2c1c2\frac{a_1}{a_2} = \frac{b_1}{b_2} \ne \frac{c_1}{c_2}, the pair is inconsistent.
- If all three ratios are equal, the pair is consistent with infinitely many solutions.

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4(i)x+y=5x + y = 5, 2x+2y=102x + 2y = 10Show solution
For x+y=5x+y=5 and 2x+2y=102x+2y=10:
a1a2=12,b1b2=12,c1c2=510=12 \frac{a_1}{a_2}=\frac{1}{2},\quad \frac{b_1}{b_2}=\frac{1}{2},\quad \frac{c_1}{c_2}=\frac{5}{10}=\frac{1}{2}
Since all three ratios are equal, the lines are coincident. Therefore, the pair is consistent and has infinitely many solutions.

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4(ii)xy=8x - y = 8, 3x3y=163x - 3y = 16Show solution
For xy=8x-y=8 and 3x3y=163x-3y=16:
a1a2=13,b1b2=13=13,c1c2=816=12 \frac{a_1}{a_2}=\frac{1}{3},\quad \frac{b_1}{b_2}=\frac{-1}{-3}=\frac{1}{3},\quad \frac{c_1}{c_2}=\frac{8}{16}=\frac{1}{2}
Since
a1a2=b1b2c1c2, \frac{a_1}{a_2}=\frac{b_1}{b_2}\ne \frac{c_1}{c_2},
the pair is inconsistent.

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4(iii)2x+y6=02x + y - 6 = 0, 4x2y4=04x - 2y - 4 = 0Show solution
For 2x+y6=02x+y-6=0 and 4x2y4=04x-2y-4=0:
a1a2=24=12,b1b2=12=12 \frac{a_1}{a_2}=\frac{2}{4}=\frac12,\quad \frac{b_1}{b_2}=\frac{1}{-2}=-\frac12
Since
a1a2b1b2, \frac{a_1}{a_2}\ne \frac{b_1}{b_2},
the lines intersect at one point, so the pair is consistent with a unique solution.

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4(iv)2x2y2=02x - 2y - 2 = 0, 4x4y5=04x - 4y - 5 = 0Show solution
For 2x2y2=02x-2y-2=0 and 4x4y5=04x-4y-5=0:
a1a2=24=12,b1b2=24=12,c1c2=25=25 \frac{a_1}{a_2}=\frac{2}{4}=\frac12,\quad \frac{b_1}{b_2}=\frac{-2}{-4}=\frac12,\quad \frac{c_1}{c_2}=\frac{-2}{-5}=\frac25
Since
a1a2=b1b2c1c2, \frac{a_1}{a_2}=\frac{b_1}{b_2}\ne \frac{c_1}{c_2},
the pair is inconsistent.

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5Half the perimeter of a rectangular garden, whose length is 4 m more than its width, is 36 m. Find the dimensions of the garden.Show solution
Let the width be xx m and the length be x+4x+4 m.

Half the perimeter is 36 m, so
length+width=36 \text{length} + \, \text{width} = 36
(x+4)+x=36 (x+4)+x=36
2x+4=36 2x+4=36
2x=32 2x=32
x=16 x=16
So width =16=16 m and length =16+4=20=16+4=20 m.

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6Given the linear equation 2x+3y8=02x + 3y - 8 = 0, write another linear equation in two variables such that the geometrical representation of the pair so formed is:Show solution
Let the given equation be 2x+3y8=02x+3y-8=0. To get a second equation:
- for intersecting lines, choose any equation whose coefficients do not make a1a2=b1b2\frac{a_1}{a_2}=\frac{b_1}{b_2};
- for parallel lines, choose another equation with a1a2=b1b2c1c2\frac{a_1}{a_2}=\frac{b_1}{b_2}\ne\frac{c_1}{c_2};
- for coincident lines, choose an equation which is a multiple of the given one.

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6(i)intersecting linesShow solution
For intersecting lines, choose a line whose coefficients do not satisfy the equality of ratios with 2x+3y8=02x+3y-8=0.

One suitable equation is:
x+y5=0 x+y-5=0
Since the ratios are not equal, the two lines will intersect at one point.

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6(ii)parallel linesShow solution
For parallel lines, the coefficients of xx and yy must be proportional, but the constant term must not be in the same ratio.

A suitable equation is:
4x+6y16=0 4x+6y-16=0
Here,
24=36=816 \frac{2}{4}=\frac{3}{6}=\frac{8}{16}
would make it coincident, so to make it parallel we need a different constant, such as 16-16? Actually for parallel lines we need
24=36816 \frac{2}{4}=\frac{3}{6}\ne \frac{8}{16}
This is not true because 816=12\frac{8}{16}=\frac12.

A correct parallel equation is:
4x+6y20=0 4x+6y-20=0
so that
24=36=12820 \frac{2}{4}=\frac{3}{6}=\frac12 \ne \frac{8}{20}
Hence the lines are parallel.

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6(iii)coincident linesShow solution
For coincident lines, choose a multiple of the given equation.

Multiplying 2x+3y8=02x+3y-8=0 by 2 gives:
4x+6y16=0 4x+6y-16=0
This represents the same line, so the pair is coincident.

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7Draw the graphs of the equations xy+1=0x - y + 1 = 0 and 3x+2y12=03x + 2y - 12 = 0. Determine the coordinates of the vertices of the triangle formed by these lines and the xx-axis, and shade the triangular region.Show solution

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EXERCISE 3.2

1Solve the following pair of linear equations by the substitution method.Show solution
Use the substitution method: express one variable from one equation and substitute into the other. Then solve for the second variable and back-substitute to get the first.

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1(i)x+y=14x + y = 14

xy=4x - y = 4
1(ii)st=3s - t = 3

s3+t2=6\frac{s}{3} + \frac{t}{2} = 6
1(iii)3xy=33x - y = 3

9x3y=99x - 3y = 9
1(iv)0.2x+0.3y=1.30.2x + 0.3y = 1.3

0.4x+0.5y=2.30.4x + 0.5y = 2.3
1(v)2x+3y=0\sqrt{2}x + \sqrt{3}y = 0

3x8y=0\sqrt{3}x - \sqrt{8}y = 0
1(vi)3x25y3=2\frac{3x}{2} - \frac{5y}{3} = -2

x3+y2=136\frac{x}{3} + \frac{y}{2} = \frac{13}{6}
2Solve 2x+3y=112x + 3y = 11 and 2x4y=242x - 4y = -24 and hence find the value of 'm' for which y=mx+3y = mx + 3.
3Form the pair of linear equations for the following problems and find their solution by substitution method.
3(i)The difference between two numbers is 26 and one number is three times the other. Find them.
3(ii)The larger of two supplementary angles exceeds the smaller by 18 degrees. Find them.
3(iii)The coach of a cricket team buys 7 bats and 6 balls for ₹ 3800. Later, she buys 3 bats and 5 balls for ₹ 1750. Find the cost of each bat and each ball.
3(iv)The taxi charges in a city consist of a fixed charge together with the charge for the distance covered. For a distance of 10 km, the charge paid is ₹ 105 and for a journey of 15 km, the charge paid is ₹ 155. What are the fixed charges and the charge per km? How much does a person have to pay for travelling a distance of 25 km?
3(v)A fraction becomes 911\frac{9}{11}, if 2 is added to both the numerator and the denominator. If, 3 is added to both the numerator and the denominator it becomes 56\frac{5}{6}. Find the fraction.
3(vi)Five years hence, the age of Jacob will be three times that of his son. Five years ago, Jacob's age was seven times that of his son. What are their present ages?

EXERCISE 3.3

1Solve the following pair of linear equations by the elimination method and the substitution method :
1(i)x+y=5x + y = 5 and 2x3y=42x - 3y = 4
1(ii)3x+4y=103x + 4y = 10 and 2x2y=22x - 2y = 2
1(iii)3x5y4=03x - 5y - 4 = 0 and 9x=2y+79x = 2y + 7
1(iv)x2+2y3=1\frac{x}{2} + \frac{2y}{3} = -1 and xy3=3x - \frac{y}{3} = 3
2Form the pair of linear equations in the following problems, and find their solutions (if they exist) by the elimination method :
2(i)If we add 1 to the numerator and subtract 1 from the denominator, a fraction reduces to 1. It becomes 12\frac{1}{2} if we only add 1 to the denominator. What is the fraction?
2(ii)Five years ago, Nuri was thrice as old as Sonu. Ten years later, Nuri will be twice as old as Sonu. How old are Nuri and Sonu?
2(iii)The sum of the digits of a two-digit number is 9. Also, nine times this number is twice the number obtained by reversing the order of the digits. Find the number.
2(iv)Meena went to a bank to withdraw ₹ 2000. She asked the cashier to give her ₹ 50 and ₹ 100 notes only. Meena got 25 notes in all. Find how many notes of ₹ 50 and ₹ 100 she received.
2(v)A lending library has a fixed charge for the first three days and an additional charge for each day thereafter. Saritha paid ₹ 27 for a book kept for seven days, while Susy paid ₹ 21 for the book she kept for five days. Find the fixed charge and the charge for each extra day.

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