SChand CLASS 9 Chapter 5 Simultaneous Linear Equations Exercise 5(A)

 Exercise 5(A)

Question 1

If (5, k) is a solution of the equation 2x + y – 7 = 0 find the value of k.

Sol :

∴(5,k) is a solution of the equations :-

⇒2x+k-7=0

⇒2×5+k-7

⇒10+k-7=0

⇒3+k=0

k=-3


Question 2

x + y = 5, x – y = 3

Sol :

⇒x+y=5...(i)

⇒x-y=3...(ii)

From (ii) x=3+y

Substituting the value of x in (i)

⇒3+y+y=5

⇒2y=5-3=

y=22=1

∴x=3+y=3+1=4

Hence x=4 , y=1


Question 3

y = 2x – 6, y = 0

Sol :

Let y=2x-6
y=0

From (i) and (ii)

⇒2x-6=0

⇒2x=6

x=62

⇒x=3

So, x=3 , y=0


Question 4

p = 2q – 1, q = 5 – 3p

Sol :

Let P=2q-1....(i)
⇒q=5-3p...(ii)

Substituting the value of p in (ii)
⇒q=5-3(2q-1)

⇒q+6q=8

⇒7q=8

q=87

From equation (ii)

p=2×871

p=1671

=1677=97

p=97,q=87


Question 5

9x + 4y = 5, 4x – 5y = 9

Sol :

9x+4y=5...(i)
4x-5y=9...(ii)

Multiplying both (i) by 5 and (ii) by 4 , we get

4x+20y=-25..(iii)

16x-20y=36...(iv)

On adding (iii) and (iv)

61x=61

x=6161=1


From (i)

⇒9×1+4y=5

⇒4y=5-9=-4

y=44=1

∴x=1 ,y=-1


Question 6

x + 3y = 5, 3x – y = 5

Sol :

x+3y=5...(i)

3x-y=5...(ii)

From (i) x=5-3y

So, we substituting the value of x in (ii)

⇒3(5-3y)-y=5

⇒15-9y-y=5

⇒-10y=5-15=-10

y=1010=1

∴x=5-3y=5-3×1=5-3=2

Hence , x=2  y=1


Question 7

3x – 7y + 10 = 0, y – 2x – 3 = 0

Sol :

3x-7y+10=0 or 
3x-7y=-10...(i)

y-2x-3=0...(ii)

From (ii) y=3+2x
Substituting the value of y in (i)

⇒3x-7(3+2x)=-10

⇒3x-21-14x=-10

⇒-11x=-10+21=11

x=1111=-1

∴y=3+2x=3+2(-1)

=3-2=1

So, x=-1 , y=1


Question 8

20u – 30v = 13, 10v – 10u = – 5

Sol :

20u-30v=13...(i)

10v-10u=-5 or 

10u-10v=5....(ii)

Multiplying (i) by 1 and (ii) by 3,  we get

20u30v=1320u20v=1020u+20v=10v=3

v=310

From (i) 10u10×[310]=5

10u+3=5

10u=5-3

u=210=15

Hence u=15,v=310


Question 9

2x – 3y = 1.3, y – x = 0.5

Sol :

2x-3y=1.3...(i)

y-x=0.5....(ii)

From (ii) y=0.5+x

Substituting the value of y in (i)

⇒2x-3(0.5+x)=1.3

⇒2x-1.5-3x=1.3

⇒-x=1.3+1.5

⇒-x=2.8

⇒x=-2.8

∴y=0.5+(-2.8)=-2.3

Hence , x=-2.8 and y=-2.3


Question 10

11x + 15y + 23 = 0, 7x – 2y – 20 = 0

Sol :

10x+15y+23=0...(i)

7x-2y-20=0....(ii)

Multiplying (i) by 2 and (ii) by 15 , then adding  we get 

22x+30y+46=0105x30y300=0127x254=0

⇒127x=254

x=254127=2

From (ii)

⇒7(2)-2y-20=0

⇒14-7y-20=0

⇒-2y=20-14=6

y=62=3

∴x=2 , y=-3


Question 11

3 – (x – 5) = y + 2

2(x + y) = 4 – 3y

Sol :

⇒3-(x-5)=y+2
⇒3-x+5=y+2
⇒-x-y=2-3-5
⇒-x-y=-6
⇒x+y=6...(i)

⇒2(x+y)=4-3y

⇒2x+2y=4-3y

⇒2x+2y+3y=4

⇒2x+5y=4...(ii)

From (i) x=6-y

Substituting the value of x in (ii)

⇒2(6-y)+5y=4

⇒12-2y+5y=4

⇒3y=4-12=-8

y=83

x=6y+83=18+83=263

Hence , x=263,y=83


Question 12

(a) x2+y4=6 , x5y2 

(b) x43=y6 , 12xy=2

Sol :

(i)

x2+y4=6....(i)

x5y2=0...(ii)

Multiplying (i) by 1 and (ii) by 12 ,  we get

x2+y4=6

x10y4=0

Adding we get

x2+x10=6

5x+x=6010

⇒6x=60

x=606=10

From (ii)

105y2=0

2y2=0

y2=2

⇒y=4

∴x=10 , y=4


(ii) 

x4y6=3...(i)

12xy=2...(ii)

Multiplying (i) 1 and (ii) by 16 , we get

x4y6=3

112xy6=26=13

On Subtracting, we get

x4x12=3+13=103

3xx=4012

⇒2x=40

x=402=20

From (ii)

202y

⇒-2=10-y=-2

⇒y=10+2=12

∴x=20 , y=12


Question 13

(a) 7x+8y=2 , 2x+12y=20

(b) 17x+16y=3 , 12x13y=5

Sol :

(i)
7x+8y=2...(i)

2x+124=20....(ii)

Multiplying (i) by 3 and (ii) by 2,  we get

21x+24y=6

4x+24y=40

Subtracting we get

17x=34

x=1734=12

From (i)

7×21+8y=2

14+8y=2

8y=2+14=16

y=816=12

x=12,y=12


(ii) 

17x+16y=3...(i)

12x13y=5...(ii)

Multiplying (i) by 1 and (ii) by 12 , we get

17x+16y=3

14x16y=52

On adding we get

17x+14x=3+52

4x+7x7x×4x=6+52

11x28x2=6+52

1128x=112

114x=11

⇒14x=1

x=114

From (i)

17×14+16y=3

147+16y=3

=2+16y=3

=16y=3-2=1=6y=1

y=16

x=114,y=16


Question 14

(a) 65x – 33p = 97, 33x – 65y = 1

(b) 23x + 31y = 77, 31x + 23y = 85

Sol :

(i) 

65x-33y=97...(i)

33x-65y=1...(ii)

So, we adding and get

98x-98y=98 [dividing both side by 98]

x-y=1...(iii)

Subtracting (2) from (1)

32x+32y=96 [dividing both sides by 32]

x+y=3...(iv)

Adding (iii) and (iv)

2x=4

x=42=2

and subtracting (iii) from (iv)

2y=2

y=22=1


(ii)

23x+31y=77...(i)

31x+23y=85...(ii)

Adding  we get

54x+54y=162...(iii)

x+y=3  [Dividing by 54]

and subtracting (i) from (ii)

8x-8y=8 [Dividing both sides by 8]

x-y=1...(iv)


Now adding (iii) and (iv) , we get

2x=4

=x=42=2

and subtracting (iv) and (iii)

2y=2

y=22=1

∴x=2 ,  y=1


Question 15

(a) 4x+6y=15 , 6x8y=14 ; y≠0

Find p, if y = px – 2

(b) 4x+5y=7 , 3x+4y=5 ; x≠0

Sol :

(i)

4x+6y=15....(i)

6x8y=14...(ii)

Multiplying (i) by 4 and (2) by 3 , we get

16x+24y=60

18x24y=42

Adding we get

34x=102

x=10234=3

From (i)

4×3+6y=15

=12+6y=17

6y=1512=3

3y=6

y=63=2

Hence , x=3 , y=2

Now, y=px-2

2=3p-2

3p=2+2=4

p=43


(ii) 

4x+5y=7...(i)

3x+4y=5...(ii)

First we will multiplying (i) by 4 and (ii) by 5 , we get

16x+20y=28

15x+20y=25

On subtracting we get

16x15x=2825

1615x=3

x=13

From (i) 

413+5y=7

=4×31+5y=7

=12+5y=7

=5y=7-12=-5

=y=55=1

50x=13, y=-1


Question 16

(a) 6x+y=7xy+3 , 12(x+y)=13(xy) , where x + y ≠ 0, x – y ≠ 0.

(b) 44x+y+30xy=10 , 55x+y+40xy=13 , where x + y ≠ 0, x – y ≠ 0.

Sol :

(a) 6x+y=7xy+3

12(x+y)=13(xy)

Let x+y=a and x-y=b then

6a=7b+3

6a7b=3...(i)

12a=13b

2a=3b...(ii)

a=32b

Substituting the value of a in (i)

632b7b=3

123b7b=3

4b7b=3

3b=3

b=33=1

a=32 b=32(1)=32

Now x+y=32

x-y=-1

Adding we get

2x=52

x=54

and subtracting 

2y=12

y=12×2=14

So, x=54,y=14


(ii)

44x+4+30xy=10

55x+y+40xy=13

Hence x+y=a , x-y=b  then

44a+30b=10...(i)

55a+40b=13...(ii)

Multiplying (i) by 4 and (ii) by 3 , we get

126a+120b=40

165a+120b=39

On Subtracting 

11a=1

a=11

From (i)

4411+30b=10

4+306=10

30b=104=61

b=306=5

Now, x+y=11

x-y=5

Adding we get

2x=16

x=162=8

and subtracting 

2y=6

y=62=3

So , x=8 , y=3


Question 17

(i) 2 (3u – v) = 5uv, 2 (w + 3v) = 5uv

(ii) 3 (a + 3b) = 11ab, 3 (2a + b) = 7ab

Sol :

(i) 2 (3u – v) = 5uv

Let 6u-2v=5uv

Dividing by uv

6v2v=5...(i)

and 2(u+3v)=5uv

2u+6v=5uv  (Dividing by uv both side)

2v+cu=5....(ii)

Multiplying both (i) by 3 and (ii) by 1

18v6u=15

2v+6v=5

Adding both sides we get

20v=20

v=2020=1

From (i) 

612u=5

2v=56=1

u=2

Hence , u=2 , v=1


(ii) 3a+9b=11ab

3b+9a=11...(i)  (Dividing both ab)

3(2a+b)=7ab

6a+3b=7ab

6b+3a=7...(ii)  (Dividing both ab)

Multiplying both (i) by 1 and (ii) by 3

3b+9a=11

18b+9a=21

Subtracting , we get

15b=10-10b=-15

b=1510=32

From (i) equation 33+9a=11

=3×23+9a=11

2+9a=11

=9a=112=9

=a=99=1

∴we get a=1 , b=32


Question 18

3x64+315y42=5y2

yx4+x87x5y3=y2x

Sol :

3x64+315y42=5y2


=3x6+1210y+8=10y4 [we take LCM of 24=4]

then

3x-10y-10=-12-8+6

3x-20y=-14...(i)

=yx4+x87x5y3=y2x

=6y6x+3x56x+40y=24y48x24

[We take LCM of 8,3=24]

=6y-6x+3x-56x+40y-24y+48x=0

=-6x+3x-56x+48x+6y+40y-24y=0

-62x+51x+46y+24y=0

-11x+22y=0

22y=11x

x=2211 

x=2y

From (i)

3x-20y=-14

3×2y-20y=-114

6y-20y=-14

-14y=-14

y=1414=1

∴x=2y=2×1=2

Hence x=2  ,y=1


Question 19

x+y=0.9 , 112(x+y)=1

Sol :

x+y=0.9=910

112(x+y)=1

x+y=112

By adding we get

2x=910+112

=9×5510=6410

x=6410×2=3210=3.2

From (i)

x-y=0.9

3.2-y=0.9

-y=0.9-3.2

-y=-2.3

y=2.3

Hence x=3.2 , y=2.3


Question 20

x2+y=0.8 , 7x+y2=10

Sol :

x2+y=0.8

x2+y=810

5x+10y=8....(i)

7x+y2=10

7=10[x+y2]

10x+5y=7

Multiplying (i) by 1 , (ii) by 2 and then subtracting

5x+10y=820x+10y=1415x=6

x=615=25

x=0.4

From (i)

5×0.4+10y=8

2.0+10y=8

10y=8-2=6

y=610=35

Hence ,x=25,y=35


Question 21

If 2x +y = 35, 3x + 4y = 65, find the value of xy

Sol :

2x+y=35...(i)

3x+4y=65...(ii)

Multiplying (i) by 4 and (ii) by 1 then subtracting

8x+4y=140

3x+4y=65

we get 5x=75

x=755=15

From (i) y=35-2x

=35-2×15

=35-30=5

xy=155=3


(a) x + y = a – b
ax – by = a² + b²

Sol :

x+y=a-b
ax-by=a2+b2

Multiplying (i) by b , (ii) by 1

bx+by=abb2

axby=a2+b2

On adding we get 

(a+b)x=a(a+b)

x=a(a+b)(a+b)=a

From (i)

a+y=a-b

y=a-b-a=-b

Hence x=a , y=-b

 

(b) ax + by = a – b … (i)

bx – ay = a + b … (ii)

Sol :

ax+by=a-b
bx-ay=a+b
Multiplying (i) by a, (ii) by b
a2x+aby=a2abb2xaby=ab+b2(a2+b2)x=a2+b2
On adding we get

x=a2+b2a2+b2=1

From (i)

a×1+by=a-b

a+by=a-b

by=a-b-a=-b

y=bb=1

Hence , x=1 , y=-1


Question 22

3 – 2(3x + 4y) = x, x34y45=2110

Sol :

3-2(3x+4y)=x
3-6x-8y=-3 or
-6x-8y-x=-3

-7x-8y=-3 or 7x+8y=3

and x34y45=2120

5x154y+16=4220

5x-4y=42+15-16

5x-4y=41

Multiplying (i) by 1 (ii) by 12 then adding

7x+8y=310x8y=8217x=85

x=8517=5

From (i)

7×5+8y=3

35+8y=3

8y=3-35=-32

y=328=4

∴x=5 , y=-4


Question 23

Can the following system of equations hold simultaneously?

3x+4y=7 , 2x+7y=5 , 5x+4y=9 . if yes, find x and y.

Sol :

3x+4y=7...(i)
2x+7y=5...(ii)
5x+4y=9...(iii)

Multiplying (i) by 2 , (ii) by 3

6x+8y=14

6x+21y=15

By adding we get

29y=29

y=2929=1

From (i)

3x+4×1=7

7x+4=7

3x=74=3

x=33=1

∴x=1 , y=1

Substituting the value of x and y in (ii)

5×1+41=9

5+4=9

9=9 is true

Yes, the system of equations are simultaneously and 

x=1 , y=1


Question 24

If the following three equations hold simultaneously for x and y, find p.

3x – 2y = 6, x3y6=12 ,  x – py = 6

Sol :

3x-2y=6...(i)

x3y6=12

2x-y=3....(ii)

Multiplying by 6

Multiplying (i) by 1 ,(ii) by 2 

3x-2y=6

4x-2y=6

On subtracting we get

-x=0

or x=0

From (i)

3×0-2y=6

-2y=6

y=62=3

∵The equations are simutaneousy

∴x=0, y=-3 will satisfy the third equation

∴x-py=6

0-p(-3)=6

3p=6

p=63=2

Hence p=2


Question 25

The sides of an equilateral triangle are (6x + 33y) cm, (8x + 9y – 5) cm and (10x + 12y – 8) respectively. Find the length of each side.

Sol :

∴Its all the sides are equal because its a equilateral triangle 

6x+3y=8x+9y-5

8x+9y-6x-3y=5

2x+6y=5...(i)

10x+12y-8x-9y=-5+8

2x+3y=3...(ii)

Multiplying (i) by 1 and (ii) by 2 On subtracting

2x+6y=54x+6y=62x=1

x=12=12

2x+6y=5

2×12+6y=5

1+6y=5

6y=5-1=4

y=46=23

Now 6x+3y

=6×12+3×23

=3+2=5

∴Each side of the triangle = 5 units

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