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 :
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 :
⇒2x-6=0
⇒2x=6
x=62
⇒x=3
So, x=3 , y=0
Question 4
p = 2q – 1, q = 5 – 3p
Sol :
⇒q+6q=8
⇒7q=8
⇒q=87
From equation (ii)
p=2×87−1
p=167−1
=16−77=97
∴p=97,q=87
Question 5
9x + 4y = 5, 4x – 5y = 9
Sol :
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=−4−4=−1
∴x=1 ,y=-1
Question 6
x + 3y = 5, 3x – y = 5
Sol :
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=−10−10=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-7(3+2x)=-10
⇒3x-21-14x=-10
⇒-11x=-10+21=11
⇒x=11−11=-1
∴y=3+2x=3+2(-1)
=3-2=1
So, x=-1 , y=1
Question 8
20u – 30v = 13, 10v – 10u = – 5
Sol :
10v-10u=-5 or
10u-10v=5....(ii)
Multiplying (i) by 1 and (ii) by 3, we get
20u−30v=1320u−20v=10−20u+20v=−−10v=3
v=−310
From (i) 10u−10×[−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 :
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 :
7x-2y-20=0....(ii)
Multiplying (i) by 2 and (ii) by 15 , then adding we get
22x+30y+46=0105x−30y−300=0127x−254=0
⇒127x=254
⇒x=254127=2
From (ii)
⇒7(2)-2y-20=0
⇒14-7y-20=0
⇒-2y=20-14=6
y=6−2=−3
∴x=2 , y=-3
Question 11
3 – (x – 5) = y + 2
2(x + y) = 4 – 3y
Sol :
⇒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=6−y+83=18+83=263
Hence , x=263,y=−83
Question 12
(a) x2+y4=6 , x5−y2
(b) x4−3=y6 , 12x−y=−2
Sol :
(i)
x5−y2=0...(ii)
Multiplying (i) by 1 and (ii) by 12 , we get
⇒x2+y4=6
⇒x10−y4=0
Adding we get
⇒x2+x10=6
⇒5x+x=6010
⇒6x=60
⇒x=606=10
From (ii)
⇒105−y2=0
⇒2−y2=0
⇒y2=2
⇒y=4
∴x=10 , y=4
(ii)
⇒x4−y6=3...(i)
⇒12x−y=−2...(ii)
Multiplying (i) 1 and (ii) by 16 , we get
⇒x4−y6=3
⇒112x−y6=−26=−13
On Subtracting, we get
⇒x4−x12=3+13=103
⇒3x−x=4012
⇒2x=40
⇒x=402=20
From (ii)
⇒202−y
⇒-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 , 12x−13y=5
Sol :
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=17−34=−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)
12x−13y=5...(ii)
Multiplying (i) by 1 and (ii) by 12 , we get
17x+16y=3
14x−16y=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 :
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 , 6x−8y=14 ; y≠0
Find p, if y = px – 2
(b) 4x+5y=7 , 3x+4y=5 ; x≠0
Sol :
6x−8y=14...(ii)
Multiplying (i) by 4 and (2) by 3 , we get
16x+24y=60
18x−24y=42
Adding we get
34x=102
x=10234=3
From (i)
4×3+6y=15
=12+6y=17
6y=15−12=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
16x−15x=28−25
16−15x=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=7x−y+3 , 12(x+y)=13(x−y) , where x + y ≠ 0, x – y ≠ 0.
(b) 44x+y+30x−y=10 , 55x+y+40x−y=13 , where x + y ≠ 0, x – y ≠ 0.
Sol :
(a) 6x+y=7x−y+3
12a=13b
2a=3b...(ii)
a=32b
Substituting the value of a in (i)
632b−7b=3
123b−7b=3
4b−7b=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+30x−y=10
55x+y+40x−y=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=10−4=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 :
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
18v−6u=15
2v+6v=5
Adding both sides we get
20v=20
v=2020=1
From (i)
61−2u=5
−2v=5−6=−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=−15−10=32
From (i) equation 33+9a=11
=3×23+9a=11
⇒2+9a=11
=9a=11−2=9
=a=99=1
∴we get a=1 , b=⋅32
Question 18
3x−64+31−5y−42=5y2
Sol :
=3x−6+12−10y+8=10y4 [we take LCM of 24=4]
then
3x-10y-10=-12-8+6
3x-20y=-14...(i)
=y−x4+x8−7x−5y3=y−2x
=6y−6x+3x−56x+40y=24y−48x24
[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=−14−14=1
∴x=2y=2×1=2
Hence x=2 ,y=1
Question 19
x+y=0.9 , 112(x+y)=1
Sol :
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
Sol :
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=14−15x=−6
x=−6−15=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 :
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
Sol :
Multiplying (i) by b , (ii) by 1
bx+by=ab−b2
ax−by=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 :
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, x−34−y−45=2110
Sol :
and x−34−y−45=2120
5x−15−4y+16=4220
5x-4y=42+15-16
5x-4y=41
Multiplying (i) by 1 (ii) by 12 then adding
7x+8y=310x−8y=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?
Sol :
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=7−4=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, x3−y6=12 , x – py = 6
Sol :
3x-2y=6...(i)
x3−y6=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=6−2=−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 :
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=6−2x=−1
x=−1−2=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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