Class 8 Exam  >  Class 8 Notes  >  Mathematics (Maths) Class 8  >  RS Aggarwal Solutions: Exercise 7B - Factorisation

RS Aggarwal Solutions: Exercise 7B - Factorisation | Mathematics (Maths) Class 8 PDF Download

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We have:
       4x
2
-9y
2
= (2x)
2
-(3y)
2
            = (2x +3y)(2x -3y)
      ? 4x
2
-9y
2
= (2x +3y)(2x -3y)            
Q u e s t i o n : 3 3
Factorise:
16a
2
 - 225b
2
S o l u t i o n :
We have:
       16a
2
-225b
2
= (4a)
2
-(15b)
2
                  = (4a +15b)(4a -15b)
      ? 16a
2
-225b
2
= (4a +15b)(4a -15b)                  
Q u e s t i o n : 3 4
Factorise:
9a
2
b
2
 - 25
S o l u t i o n :
We have:
9a
2
b
2
-25 = (3ab)
2
-(5)
2
               = (3ab +5)(3ab -5)       
     ? 9a
2
b
2
-25 = (3ab +5)(3ab -5)               
Q u e s t i o n : 3 5
Factorise:
16a
2
 - 144
S o l u t i o n :
 We have:
         16a
2
-144 = (4a)
2
-(12)
2
               = (4a +12)(4a -12)                = 4 a + 3 4 a -3 = 16 a +3 a -3
       ? 16a
2
-144 = 16(a +3)(a -3)               
Q u e s t i o n : 3 6
Factorise:
63a
2
 - 112b
2
S o l u t i o n :
 We have:
        63a
2
-112b
2
= 7 9a
2
-16b
2
                   = 7 (3a)
2
-(4b)
2
                   = 7(3a +4b)(3a -4b)
       ? 63a
2
-112b
2
= 7(3a +4b)(3a -4b)                   
Q u e s t i o n : 3 7
Factorise:
20a
2
 - 45b
2
S o l u t i o n :
 We have:
       20a
2
-45b
2
= 5 4a
2
-9b
2
                     = 5 (2a)
2
-(3b)
2
                     = 5(2a +3b)(2a -3b)
      ? 20a
2
-45b
2
= 5(2a +3b)(2a -3b)                     
Q u e s t i o n : 3 8
Factorise:
12x
2
 - 27
S o l u t i o n :
We have:
       12x
2
-27 = 3 4x
2
-9                 = 3 (2x)
2
-(3)
2
                 = 3(2x +3)(2x -3)
      ? 12x
2
-27 = 3(2x +3)(2x -3)                 
Q u e s t i o n : 3 9
Factorise:
x
3
 - 64x
S o l u t i o n :
We have:
       x
3
-64x = x x
2
-64              = x (x)
2
-(8)
2
              = x(x +8)(x -8)
      ? x
3
-64x = x(x +8)(x -8)              
Q u e s t i o n : 4 0
Factorise:
16x
5
 - 144x
3
S o l u t i o n :
We have:
        16x
5
-144x
3
= 16x
3
x
2
-9                       = 16x
3
(x)
2
-(3)
2
                       = 16x
3
(x +3)(x -3)  
       ? 16x
5
-144x
3
= 16x
3
(x +3)(x -3)                         
Q u e s t i o n : 4 1
( ) ( ) ( ) ( )
( ) { }
( ) { }
( ) { }
( ) { }
( ) { }
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We have:
       4x
2
-9y
2
= (2x)
2
-(3y)
2
            = (2x +3y)(2x -3y)
      ? 4x
2
-9y
2
= (2x +3y)(2x -3y)            
Q u e s t i o n : 3 3
Factorise:
16a
2
 - 225b
2
S o l u t i o n :
We have:
       16a
2
-225b
2
= (4a)
2
-(15b)
2
                  = (4a +15b)(4a -15b)
      ? 16a
2
-225b
2
= (4a +15b)(4a -15b)                  
Q u e s t i o n : 3 4
Factorise:
9a
2
b
2
 - 25
S o l u t i o n :
We have:
9a
2
b
2
-25 = (3ab)
2
-(5)
2
               = (3ab +5)(3ab -5)       
     ? 9a
2
b
2
-25 = (3ab +5)(3ab -5)               
Q u e s t i o n : 3 5
Factorise:
16a
2
 - 144
S o l u t i o n :
 We have:
         16a
2
-144 = (4a)
2
-(12)
2
               = (4a +12)(4a -12)                = 4 a + 3 4 a -3 = 16 a +3 a -3
       ? 16a
2
-144 = 16(a +3)(a -3)               
Q u e s t i o n : 3 6
Factorise:
63a
2
 - 112b
2
S o l u t i o n :
 We have:
        63a
2
-112b
2
= 7 9a
2
-16b
2
                   = 7 (3a)
2
-(4b)
2
                   = 7(3a +4b)(3a -4b)
       ? 63a
2
-112b
2
= 7(3a +4b)(3a -4b)                   
Q u e s t i o n : 3 7
Factorise:
20a
2
 - 45b
2
S o l u t i o n :
 We have:
       20a
2
-45b
2
= 5 4a
2
-9b
2
                     = 5 (2a)
2
-(3b)
2
                     = 5(2a +3b)(2a -3b)
      ? 20a
2
-45b
2
= 5(2a +3b)(2a -3b)                     
Q u e s t i o n : 3 8
Factorise:
12x
2
 - 27
S o l u t i o n :
We have:
       12x
2
-27 = 3 4x
2
-9                 = 3 (2x)
2
-(3)
2
                 = 3(2x +3)(2x -3)
      ? 12x
2
-27 = 3(2x +3)(2x -3)                 
Q u e s t i o n : 3 9
Factorise:
x
3
 - 64x
S o l u t i o n :
We have:
       x
3
-64x = x x
2
-64              = x (x)
2
-(8)
2
              = x(x +8)(x -8)
      ? x
3
-64x = x(x +8)(x -8)              
Q u e s t i o n : 4 0
Factorise:
16x
5
 - 144x
3
S o l u t i o n :
We have:
        16x
5
-144x
3
= 16x
3
x
2
-9                       = 16x
3
(x)
2
-(3)
2
                       = 16x
3
(x +3)(x -3)  
       ? 16x
5
-144x
3
= 16x
3
(x +3)(x -3)                         
Q u e s t i o n : 4 1
( ) ( ) ( ) ( )
( ) { }
( ) { }
( ) { }
( ) { }
( ) { }
Factorise:
3x
5
 - 48x
3
S o l u t i o n :
 We have:
      3x
5
-48x
3
= 3x
3
x
2
-16                  = 3x
3
(x)
2
-(4)
2
                  = 3x
3
(x +4)(x -4)
      ? 3x
5
-48x
3
= 3x
3
(x +4)(x -4)                  
Q u e s t i o n : 4 2
Factorise:
16p
3
 - 4p
S o l u t i o n :
We have:
      16p
3
-4p = 4p 4p
2
-1                 = 4p (2p)
2
-(1)
2
                 = 4p(2p +1)(2p -1)
      ? 16p
3
-4p = 4p(2p +1)(2p -1)                 
Q u e s t i o n : 4 3
Factorise:
63a
2
b
2
 - 7
S o l u t i o n :
 We have:
      63a
2
b
2
-7 = 7 9a
2
b
2
-1                   = 7 (3ab)
2
-(1)
2
                   = 7(3ab +1)(3ab -1)
     ? 63a
2
b
2
-7 = 7(3ab +1)(3ab -1)                   
Q u e s t i o n : 4 4
Factorise:
1 - b -c
2
S o l u t i o n :
 We have:
      1 -(b -c)
2
= (1)
2
-(b -c)
2
                   = {1 +(b -c)}{1 -(b -c)}                   = (1 +b -c)(1 -b +c)
      ? 1 -(b -c)
2
= (1 +b -c)(1 -b +c)                   
Q u e s t i o n : 4 5
Factorise:
(2a + 3b)
2
 - 16c
2
S o l u t i o n :
We have:
     (2a +3b)
2
-16c
2
= (2a +3b)
2
-(4c)
2
                               = {(2a +3b)+4c}{(2a +3b)-4c}                               = (2a +3b +4c)(2a +3b -4c)
     ? (2a +3b)
2
-16c
2
= (2a +3b +4c)(2a +3b -4c)                               
Q u e s t i o n : 4 6
Factorise:
(l + m)
2
 - (l - m)
2
S o l u t i o n :
We have:
       (l +m)
2
-(l -m)
2
= {(l +m)+(l -m)}{(l +m)-(l -m)}                                = (l +m +l -m)(l +m -l +m)                                = (2l)(2m)
      ? (l +m)
2
-(l -m)
2
= (2l)(2m)                                
Q u e s t i o n : 4 7
Factorise:
(2x + 5y)
2
 - 1
S o l u t i o n :
We have:
      (2x +5y)
2
-1 = (2x +5y)
2
-(1)
2
                        = {(2x +5y)+1}{(2x +5y)-1}                        = (2x +5y +1)(2x +5y -1)
      ? (2x +5y)
2
-1 = (2x +5y +1)(2x +5y -1)                        
Q u e s t i o n : 4 8
Factorise:
36c
2
 - (5a + b)
2
S o l u t i o n :
 We have:
     36c
2
-(5a +b)
2
= (6c)
2
-(5a +b)
2
                            = {(6c)+(5a +b)}{(6c)-(5a +b)}                            = (6c +5a +b)(6c -5a -b)
      ? 36c
2
-(5a +b)
2
= (6c +5a +b)(6c -5a -b)                            
Q u e s t i o n : 4 9
Factorise:
(3x - 4y)
2
 - 25z
2
S o l u t i o n :
We have:
       (3x -4y)
2
-25z
2
= (3x -4y)
2
-(5z)
2
                          = {(3x -4y)+5z}{(3x -4y)-5z}                          = (3x -4y +5z)(3x -4y -5z)
      ? (3x -4y)
2
-25z
2
= (3x -4y +5z)(3x -4y -5z)                        
( ) { }
( ) { }
( ) { }
Page 4


      
 
        
                                                                
                                        
      
 
                                                                                  
                                
      
 
                                                  
                             
      
 
      
                                                                                                      
       
      
 
        
                                       
       
  
 
                
                 
  
  
                
                 
  
 
                    
                    
  
We have:
       4x
2
-9y
2
= (2x)
2
-(3y)
2
            = (2x +3y)(2x -3y)
      ? 4x
2
-9y
2
= (2x +3y)(2x -3y)            
Q u e s t i o n : 3 3
Factorise:
16a
2
 - 225b
2
S o l u t i o n :
We have:
       16a
2
-225b
2
= (4a)
2
-(15b)
2
                  = (4a +15b)(4a -15b)
      ? 16a
2
-225b
2
= (4a +15b)(4a -15b)                  
Q u e s t i o n : 3 4
Factorise:
9a
2
b
2
 - 25
S o l u t i o n :
We have:
9a
2
b
2
-25 = (3ab)
2
-(5)
2
               = (3ab +5)(3ab -5)       
     ? 9a
2
b
2
-25 = (3ab +5)(3ab -5)               
Q u e s t i o n : 3 5
Factorise:
16a
2
 - 144
S o l u t i o n :
 We have:
         16a
2
-144 = (4a)
2
-(12)
2
               = (4a +12)(4a -12)                = 4 a + 3 4 a -3 = 16 a +3 a -3
       ? 16a
2
-144 = 16(a +3)(a -3)               
Q u e s t i o n : 3 6
Factorise:
63a
2
 - 112b
2
S o l u t i o n :
 We have:
        63a
2
-112b
2
= 7 9a
2
-16b
2
                   = 7 (3a)
2
-(4b)
2
                   = 7(3a +4b)(3a -4b)
       ? 63a
2
-112b
2
= 7(3a +4b)(3a -4b)                   
Q u e s t i o n : 3 7
Factorise:
20a
2
 - 45b
2
S o l u t i o n :
 We have:
       20a
2
-45b
2
= 5 4a
2
-9b
2
                     = 5 (2a)
2
-(3b)
2
                     = 5(2a +3b)(2a -3b)
      ? 20a
2
-45b
2
= 5(2a +3b)(2a -3b)                     
Q u e s t i o n : 3 8
Factorise:
12x
2
 - 27
S o l u t i o n :
We have:
       12x
2
-27 = 3 4x
2
-9                 = 3 (2x)
2
-(3)
2
                 = 3(2x +3)(2x -3)
      ? 12x
2
-27 = 3(2x +3)(2x -3)                 
Q u e s t i o n : 3 9
Factorise:
x
3
 - 64x
S o l u t i o n :
We have:
       x
3
-64x = x x
2
-64              = x (x)
2
-(8)
2
              = x(x +8)(x -8)
      ? x
3
-64x = x(x +8)(x -8)              
Q u e s t i o n : 4 0
Factorise:
16x
5
 - 144x
3
S o l u t i o n :
We have:
        16x
5
-144x
3
= 16x
3
x
2
-9                       = 16x
3
(x)
2
-(3)
2
                       = 16x
3
(x +3)(x -3)  
       ? 16x
5
-144x
3
= 16x
3
(x +3)(x -3)                         
Q u e s t i o n : 4 1
( ) ( ) ( ) ( )
( ) { }
( ) { }
( ) { }
( ) { }
( ) { }
Factorise:
3x
5
 - 48x
3
S o l u t i o n :
 We have:
      3x
5
-48x
3
= 3x
3
x
2
-16                  = 3x
3
(x)
2
-(4)
2
                  = 3x
3
(x +4)(x -4)
      ? 3x
5
-48x
3
= 3x
3
(x +4)(x -4)                  
Q u e s t i o n : 4 2
Factorise:
16p
3
 - 4p
S o l u t i o n :
We have:
      16p
3
-4p = 4p 4p
2
-1                 = 4p (2p)
2
-(1)
2
                 = 4p(2p +1)(2p -1)
      ? 16p
3
-4p = 4p(2p +1)(2p -1)                 
Q u e s t i o n : 4 3
Factorise:
63a
2
b
2
 - 7
S o l u t i o n :
 We have:
      63a
2
b
2
-7 = 7 9a
2
b
2
-1                   = 7 (3ab)
2
-(1)
2
                   = 7(3ab +1)(3ab -1)
     ? 63a
2
b
2
-7 = 7(3ab +1)(3ab -1)                   
Q u e s t i o n : 4 4
Factorise:
1 - b -c
2
S o l u t i o n :
 We have:
      1 -(b -c)
2
= (1)
2
-(b -c)
2
                   = {1 +(b -c)}{1 -(b -c)}                   = (1 +b -c)(1 -b +c)
      ? 1 -(b -c)
2
= (1 +b -c)(1 -b +c)                   
Q u e s t i o n : 4 5
Factorise:
(2a + 3b)
2
 - 16c
2
S o l u t i o n :
We have:
     (2a +3b)
2
-16c
2
= (2a +3b)
2
-(4c)
2
                               = {(2a +3b)+4c}{(2a +3b)-4c}                               = (2a +3b +4c)(2a +3b -4c)
     ? (2a +3b)
2
-16c
2
= (2a +3b +4c)(2a +3b -4c)                               
Q u e s t i o n : 4 6
Factorise:
(l + m)
2
 - (l - m)
2
S o l u t i o n :
We have:
       (l +m)
2
-(l -m)
2
= {(l +m)+(l -m)}{(l +m)-(l -m)}                                = (l +m +l -m)(l +m -l +m)                                = (2l)(2m)
      ? (l +m)
2
-(l -m)
2
= (2l)(2m)                                
Q u e s t i o n : 4 7
Factorise:
(2x + 5y)
2
 - 1
S o l u t i o n :
We have:
      (2x +5y)
2
-1 = (2x +5y)
2
-(1)
2
                        = {(2x +5y)+1}{(2x +5y)-1}                        = (2x +5y +1)(2x +5y -1)
      ? (2x +5y)
2
-1 = (2x +5y +1)(2x +5y -1)                        
Q u e s t i o n : 4 8
Factorise:
36c
2
 - (5a + b)
2
S o l u t i o n :
 We have:
     36c
2
-(5a +b)
2
= (6c)
2
-(5a +b)
2
                            = {(6c)+(5a +b)}{(6c)-(5a +b)}                            = (6c +5a +b)(6c -5a -b)
      ? 36c
2
-(5a +b)
2
= (6c +5a +b)(6c -5a -b)                            
Q u e s t i o n : 4 9
Factorise:
(3x - 4y)
2
 - 25z
2
S o l u t i o n :
We have:
       (3x -4y)
2
-25z
2
= (3x -4y)
2
-(5z)
2
                          = {(3x -4y)+5z}{(3x -4y)-5z}                          = (3x -4y +5z)(3x -4y -5z)
      ? (3x -4y)
2
-25z
2
= (3x -4y +5z)(3x -4y -5z)                        
( ) { }
( ) { }
( ) { }
Q u e s t i o n : 5 0
Factorise:
x
2
 - y
2
 - 2y - 1
S o l u t i o n :
 We have:
      x
2
-y
2
-2y -1 = x
2
- y
2
+2y +1   
                      = (x)
2
-(y +1)
2
= {x +(y +1)}{x -(y +1)} = (x +y +1)(x -y -1)
      ? x
2
-y
2
-2y -1 = (x +y +1)(x -y -1)   
Q u e s t i o n : 5 1
Factorise:
25 - a
2
 - b
2
 - 2ab
S o l u t i o n :
We have:
      25 -a
2
-b
2
-2ab = 25 - a
2
+b
2
+2ab
                                = 25 -(a +b)
2
= (5)
2
-(a +b)
2
= {5 +(a +b)}{5 -(a +b)} = (5 +a +b)(5 -a -b)
      ? 25 -a
2
-b
2
-2ab = (5 +a +b)(5 -a -b)
Q u e s t i o n : 5 2
Factorise:
25a
2
 - 4b
2
 + 28bc - 49c
2
S o l u t i o n :
 We have:
       25a
2
-4b
2
+28bc -49c
2
= 25a
2
- 4b
2
-28bc +49c
2
                                           = (5a)
2
-(2b -7c)
2
= {5a +(2b -7c)}{5a -(2b -7c)} = (5a +2b -7c)(5a -2b +7c)
     ? 25a
2
-4b
2
+28bc -49c
2
= (5a +2b -7c)(5a -2b +7c)
Q u e s t i o n : 5 3
Factorise:
9a
2
 - b
2
 + 4b - 4
S o l u t i o n :
We have:
      9a
2
-b
2
+4b -4 = 9a
2
- b
2
-4b +4
                             = (3a)
2
-(b -2)
2
= {3a +(b -2)}{3a -(b -2)} = (3a +b -2)(3a -b +2)
     ? 9a
2
-b
2
+4b -4 = (3a +b -2)(3a -b +2)
Q u e s t i o n : 5 4
Factorise:
100 - (x - 5)
2
S o l u t i o n :
We have:
       100 -(x -5)
2
= (10)
2
-(x -5)
2
                          = {10 +(x -5)}{10 -(x -5)} = (10 +x -5)(10 -x +5) = (5 +x)(15 -x)
      ? 100 -(x -5)
2
= (5 +x)(15 -x)
Q u e s t i o n : 5 5
Factorise:
Evaluate {405
2
 - 395
2
}.
S o l u t i o n :
 We have:
       (405)
2
-(395)
2
= (405 +395)(405 -395)                              = (800 ×10)                              = 8000
      ? (405)
2
-(395)
2
= 8000                              
Q u e s t i o n : 5 6
Factorise:
Evaluate {7.8
2
 - 2.2
2
}.
S o l u t i o n :
We have:
      (7. 8)
2
-(2. 2)
2
= (7. 8 +2. 2)(7. 8 -2. 2)                            = (10 ×5. 6)                            = 56
      ? (7. 8)
2
-(2. 2)
2
= 56
    
 
                         
                          
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