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Mensuration – I (Perimeter and Area of Rectilinear Figures Exercise 20.3) RD Sharma Solutions | Mathematics (Maths) Class 7 PDF Download

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1. Find the area of a parallelogram with base 8 cm and altitude 4.5 cm. 
 
Solution: 
Given base = 8 cm and altitude = 4.5 cm 
We know that area of the parallelogram = Base x Altitude 
= 8 cm x 4.5 cm 
Therefore, area of parallelogram = 36 cm
2
 
 
2. Find the area in square meters of the parallelogram whose base and altitudes are as 
under 
(i) Base =15 dm, altitude = 6.4 dm 
(ii) Base =1 m 40 cm, altitude = 60 cm 
 
Solution: 
(i) Given base =15 dm, altitude = 6.4 dm 
By converting these to standard form we get, 
Base = 15 dm = (15 x 10) cm = 150 cm = 1.5 m 
Altitude = 6.4 dm = (6.4 x 10) cm = 64 cm = 0.64 m 
We know that area of the parallelogram = Base x Altitude 
= 1.5 m x 0.64 m 
Area of parallelogram = 0.96 m
2
 
 
(ii) Given base = 1 m 40 cm = 1.4 m [Since 100 cm = 1 m] 
Altitude = 60 cm = 0.6 m [Since 100 cm = 1 m] 
 We know that area of the parallelogram = Base x Altitude 
= 1.4 m x 0.6 m 
= 0.84 m
2
      
 
3.  Find the altitude of a parallelogram whose area is 54 d m
2
 and base is 12 dm. 
 
Solution: 
Given area of the given parallelogram = 54 d m
2
 
Base of the given parallelogram = 12 dm 
We know that area of the parallelogram = Base x Altitude 
Page 2


 
 
 
 
 
  
 
 
        
 
1. Find the area of a parallelogram with base 8 cm and altitude 4.5 cm. 
 
Solution: 
Given base = 8 cm and altitude = 4.5 cm 
We know that area of the parallelogram = Base x Altitude 
= 8 cm x 4.5 cm 
Therefore, area of parallelogram = 36 cm
2
 
 
2. Find the area in square meters of the parallelogram whose base and altitudes are as 
under 
(i) Base =15 dm, altitude = 6.4 dm 
(ii) Base =1 m 40 cm, altitude = 60 cm 
 
Solution: 
(i) Given base =15 dm, altitude = 6.4 dm 
By converting these to standard form we get, 
Base = 15 dm = (15 x 10) cm = 150 cm = 1.5 m 
Altitude = 6.4 dm = (6.4 x 10) cm = 64 cm = 0.64 m 
We know that area of the parallelogram = Base x Altitude 
= 1.5 m x 0.64 m 
Area of parallelogram = 0.96 m
2
 
 
(ii) Given base = 1 m 40 cm = 1.4 m [Since 100 cm = 1 m] 
Altitude = 60 cm = 0.6 m [Since 100 cm = 1 m] 
 We know that area of the parallelogram = Base x Altitude 
= 1.4 m x 0.6 m 
= 0.84 m
2
      
 
3.  Find the altitude of a parallelogram whose area is 54 d m
2
 and base is 12 dm. 
 
Solution: 
Given area of the given parallelogram = 54 d m
2
 
Base of the given parallelogram = 12 dm 
We know that area of the parallelogram = Base x Altitude 
 
 
 
 
 
  
 
 
Therefore altitude of the given parallelogram = Area/Base  
= 54/12 dm  
= 4.5 dm 
 
4. The area of a rhombus is 28 m
2
. If its perimeter be 28 m, find its altitude. 
 
Solution: 
Given perimeter of a rhombus = 28 m  
But we know that perimeter of a rhombus = 4 (Side) 
4(Side) = 28 m  
Side = 28/4 
Side = 7m 
Now, Area of the rhombus = 28 m
2
 
But we know that area of rhombus = Side x Altitude 
(Side x Altitude) = 28 m
2
  
(7 x Altitude) = 28 m
2
 
Altitude = 28/7 = 4 m 
 
5. In Fig. 20, ABCD is a parallelogram, DL ? AB and DM ? BC. If AB = 18 cm, BC =12 cm 
and DM= 9.3 cm, find DL. 
 
Solution: 
Given DL ? AB and DM ? BC 
Taking BC as the base, BC = 12 cm and altitude DM = 9.3 cm 
We know that area of parallelogram ABCD = Base x Altitude  
= (12 cm x 9.3 cm)  
Page 3


 
 
 
 
 
  
 
 
        
 
1. Find the area of a parallelogram with base 8 cm and altitude 4.5 cm. 
 
Solution: 
Given base = 8 cm and altitude = 4.5 cm 
We know that area of the parallelogram = Base x Altitude 
= 8 cm x 4.5 cm 
Therefore, area of parallelogram = 36 cm
2
 
 
2. Find the area in square meters of the parallelogram whose base and altitudes are as 
under 
(i) Base =15 dm, altitude = 6.4 dm 
(ii) Base =1 m 40 cm, altitude = 60 cm 
 
Solution: 
(i) Given base =15 dm, altitude = 6.4 dm 
By converting these to standard form we get, 
Base = 15 dm = (15 x 10) cm = 150 cm = 1.5 m 
Altitude = 6.4 dm = (6.4 x 10) cm = 64 cm = 0.64 m 
We know that area of the parallelogram = Base x Altitude 
= 1.5 m x 0.64 m 
Area of parallelogram = 0.96 m
2
 
 
(ii) Given base = 1 m 40 cm = 1.4 m [Since 100 cm = 1 m] 
Altitude = 60 cm = 0.6 m [Since 100 cm = 1 m] 
 We know that area of the parallelogram = Base x Altitude 
= 1.4 m x 0.6 m 
= 0.84 m
2
      
 
3.  Find the altitude of a parallelogram whose area is 54 d m
2
 and base is 12 dm. 
 
Solution: 
Given area of the given parallelogram = 54 d m
2
 
Base of the given parallelogram = 12 dm 
We know that area of the parallelogram = Base x Altitude 
 
 
 
 
 
  
 
 
Therefore altitude of the given parallelogram = Area/Base  
= 54/12 dm  
= 4.5 dm 
 
4. The area of a rhombus is 28 m
2
. If its perimeter be 28 m, find its altitude. 
 
Solution: 
Given perimeter of a rhombus = 28 m  
But we know that perimeter of a rhombus = 4 (Side) 
4(Side) = 28 m  
Side = 28/4 
Side = 7m 
Now, Area of the rhombus = 28 m
2
 
But we know that area of rhombus = Side x Altitude 
(Side x Altitude) = 28 m
2
  
(7 x Altitude) = 28 m
2
 
Altitude = 28/7 = 4 m 
 
5. In Fig. 20, ABCD is a parallelogram, DL ? AB and DM ? BC. If AB = 18 cm, BC =12 cm 
and DM= 9.3 cm, find DL. 
 
Solution: 
Given DL ? AB and DM ? BC 
Taking BC as the base, BC = 12 cm and altitude DM = 9.3 cm 
We know that area of parallelogram ABCD = Base x Altitude  
= (12 cm x 9.3 cm)  
 
 
 
 
 
  
 
 
= 111.6 c m
2
 ….. Equation (i) 
Now, by taking AB as the base, 
We have, Area of the parallelogram ABCD = Base x Altitude 
= (18 cm x DL) ….. Equation (ii) 
From (i) and (ii), we have  
18 cm x DL = 111.6 c m
2
 
DL = 111.6/18  
= 6.2 cm 
 
6. The longer side of a parallelogram is 54 cm and the corresponding altitude is 16 cm. 
If the altitude corresponding to the shorter side is 24 cm, find the length of the shorter 
side. 
 
Solution: 
 
Let ABCD is a parallelogram with the longer side AB = 54 cm and corresponding altitude 
AE = 16 cm.  
The shorter side is BC and the corresponding altitude is CF = 24 cm. 
We know that area of a parallelogram = base x height. 
We have two altitudes and two corresponding bases. 
By equating them we get, 
½ x BC x CF = ½ x AB x AE 
On simplifying, we get 
BC x CF = AB x AE 
BC x 24 = 54 x 16 
BC = (54 × 16)/24  
= 36 cm 
Hence, the length of the shorter side BC = AD = 36 cm. 
 
7. In Fig. 21, ABCD is a parallelogram, DL ? AB. If AB = 20 cm, AD = 13 cm and area of 
Page 4


 
 
 
 
 
  
 
 
        
 
1. Find the area of a parallelogram with base 8 cm and altitude 4.5 cm. 
 
Solution: 
Given base = 8 cm and altitude = 4.5 cm 
We know that area of the parallelogram = Base x Altitude 
= 8 cm x 4.5 cm 
Therefore, area of parallelogram = 36 cm
2
 
 
2. Find the area in square meters of the parallelogram whose base and altitudes are as 
under 
(i) Base =15 dm, altitude = 6.4 dm 
(ii) Base =1 m 40 cm, altitude = 60 cm 
 
Solution: 
(i) Given base =15 dm, altitude = 6.4 dm 
By converting these to standard form we get, 
Base = 15 dm = (15 x 10) cm = 150 cm = 1.5 m 
Altitude = 6.4 dm = (6.4 x 10) cm = 64 cm = 0.64 m 
We know that area of the parallelogram = Base x Altitude 
= 1.5 m x 0.64 m 
Area of parallelogram = 0.96 m
2
 
 
(ii) Given base = 1 m 40 cm = 1.4 m [Since 100 cm = 1 m] 
Altitude = 60 cm = 0.6 m [Since 100 cm = 1 m] 
 We know that area of the parallelogram = Base x Altitude 
= 1.4 m x 0.6 m 
= 0.84 m
2
      
 
3.  Find the altitude of a parallelogram whose area is 54 d m
2
 and base is 12 dm. 
 
Solution: 
Given area of the given parallelogram = 54 d m
2
 
Base of the given parallelogram = 12 dm 
We know that area of the parallelogram = Base x Altitude 
 
 
 
 
 
  
 
 
Therefore altitude of the given parallelogram = Area/Base  
= 54/12 dm  
= 4.5 dm 
 
4. The area of a rhombus is 28 m
2
. If its perimeter be 28 m, find its altitude. 
 
Solution: 
Given perimeter of a rhombus = 28 m  
But we know that perimeter of a rhombus = 4 (Side) 
4(Side) = 28 m  
Side = 28/4 
Side = 7m 
Now, Area of the rhombus = 28 m
2
 
But we know that area of rhombus = Side x Altitude 
(Side x Altitude) = 28 m
2
  
(7 x Altitude) = 28 m
2
 
Altitude = 28/7 = 4 m 
 
5. In Fig. 20, ABCD is a parallelogram, DL ? AB and DM ? BC. If AB = 18 cm, BC =12 cm 
and DM= 9.3 cm, find DL. 
 
Solution: 
Given DL ? AB and DM ? BC 
Taking BC as the base, BC = 12 cm and altitude DM = 9.3 cm 
We know that area of parallelogram ABCD = Base x Altitude  
= (12 cm x 9.3 cm)  
 
 
 
 
 
  
 
 
= 111.6 c m
2
 ….. Equation (i) 
Now, by taking AB as the base, 
We have, Area of the parallelogram ABCD = Base x Altitude 
= (18 cm x DL) ….. Equation (ii) 
From (i) and (ii), we have  
18 cm x DL = 111.6 c m
2
 
DL = 111.6/18  
= 6.2 cm 
 
6. The longer side of a parallelogram is 54 cm and the corresponding altitude is 16 cm. 
If the altitude corresponding to the shorter side is 24 cm, find the length of the shorter 
side. 
 
Solution: 
 
Let ABCD is a parallelogram with the longer side AB = 54 cm and corresponding altitude 
AE = 16 cm.  
The shorter side is BC and the corresponding altitude is CF = 24 cm. 
We know that area of a parallelogram = base x height. 
We have two altitudes and two corresponding bases. 
By equating them we get, 
½ x BC x CF = ½ x AB x AE 
On simplifying, we get 
BC x CF = AB x AE 
BC x 24 = 54 x 16 
BC = (54 × 16)/24  
= 36 cm 
Hence, the length of the shorter side BC = AD = 36 cm. 
 
7. In Fig. 21, ABCD is a parallelogram, DL ? AB. If AB = 20 cm, AD = 13 cm and area of 
 
 
 
 
 
  
 
 
the parallelogram is 100 c m
2
, find AL. 
 
Solution: 
From the figure we have ABCD is a parallelogram with base AB = 20 cm and 
corresponding altitude DL. 
It is given that the area of the parallelogram ABCD = 100 c m
2
 
We know that the area of a parallelogram = Base x Height 
Therefore, 
100 = AB x DL 
100 = 20 x DL 
DL = 100/20 = 5 cm 
By observing the picture it is clear that we have to apply the Pythagoras theorem, 
Therefore by Pythagoras theorem, we have, 
(AD)
2 
= (AL)
2
 + (DL)
2
 
(13)
2
 = (AL)
2
+ (5)
2
 
(AL)
2
 = (13)
2
 – (5)
2
 
(AL)
2
 = 169 – 25  
= 144 
We know that 12
2 
= 144 
(AL)
2
 = (12)
2
 
AL = 12 cm 
Hence, length of AL is 12 cm. 
 
8. In Fig. 21, if AB = 35 cm, AD= 20 cm and area of the parallelogram is 560 cm
2
, find 
LB. 
Page 5


 
 
 
 
 
  
 
 
        
 
1. Find the area of a parallelogram with base 8 cm and altitude 4.5 cm. 
 
Solution: 
Given base = 8 cm and altitude = 4.5 cm 
We know that area of the parallelogram = Base x Altitude 
= 8 cm x 4.5 cm 
Therefore, area of parallelogram = 36 cm
2
 
 
2. Find the area in square meters of the parallelogram whose base and altitudes are as 
under 
(i) Base =15 dm, altitude = 6.4 dm 
(ii) Base =1 m 40 cm, altitude = 60 cm 
 
Solution: 
(i) Given base =15 dm, altitude = 6.4 dm 
By converting these to standard form we get, 
Base = 15 dm = (15 x 10) cm = 150 cm = 1.5 m 
Altitude = 6.4 dm = (6.4 x 10) cm = 64 cm = 0.64 m 
We know that area of the parallelogram = Base x Altitude 
= 1.5 m x 0.64 m 
Area of parallelogram = 0.96 m
2
 
 
(ii) Given base = 1 m 40 cm = 1.4 m [Since 100 cm = 1 m] 
Altitude = 60 cm = 0.6 m [Since 100 cm = 1 m] 
 We know that area of the parallelogram = Base x Altitude 
= 1.4 m x 0.6 m 
= 0.84 m
2
      
 
3.  Find the altitude of a parallelogram whose area is 54 d m
2
 and base is 12 dm. 
 
Solution: 
Given area of the given parallelogram = 54 d m
2
 
Base of the given parallelogram = 12 dm 
We know that area of the parallelogram = Base x Altitude 
 
 
 
 
 
  
 
 
Therefore altitude of the given parallelogram = Area/Base  
= 54/12 dm  
= 4.5 dm 
 
4. The area of a rhombus is 28 m
2
. If its perimeter be 28 m, find its altitude. 
 
Solution: 
Given perimeter of a rhombus = 28 m  
But we know that perimeter of a rhombus = 4 (Side) 
4(Side) = 28 m  
Side = 28/4 
Side = 7m 
Now, Area of the rhombus = 28 m
2
 
But we know that area of rhombus = Side x Altitude 
(Side x Altitude) = 28 m
2
  
(7 x Altitude) = 28 m
2
 
Altitude = 28/7 = 4 m 
 
5. In Fig. 20, ABCD is a parallelogram, DL ? AB and DM ? BC. If AB = 18 cm, BC =12 cm 
and DM= 9.3 cm, find DL. 
 
Solution: 
Given DL ? AB and DM ? BC 
Taking BC as the base, BC = 12 cm and altitude DM = 9.3 cm 
We know that area of parallelogram ABCD = Base x Altitude  
= (12 cm x 9.3 cm)  
 
 
 
 
 
  
 
 
= 111.6 c m
2
 ….. Equation (i) 
Now, by taking AB as the base, 
We have, Area of the parallelogram ABCD = Base x Altitude 
= (18 cm x DL) ….. Equation (ii) 
From (i) and (ii), we have  
18 cm x DL = 111.6 c m
2
 
DL = 111.6/18  
= 6.2 cm 
 
6. The longer side of a parallelogram is 54 cm and the corresponding altitude is 16 cm. 
If the altitude corresponding to the shorter side is 24 cm, find the length of the shorter 
side. 
 
Solution: 
 
Let ABCD is a parallelogram with the longer side AB = 54 cm and corresponding altitude 
AE = 16 cm.  
The shorter side is BC and the corresponding altitude is CF = 24 cm. 
We know that area of a parallelogram = base x height. 
We have two altitudes and two corresponding bases. 
By equating them we get, 
½ x BC x CF = ½ x AB x AE 
On simplifying, we get 
BC x CF = AB x AE 
BC x 24 = 54 x 16 
BC = (54 × 16)/24  
= 36 cm 
Hence, the length of the shorter side BC = AD = 36 cm. 
 
7. In Fig. 21, ABCD is a parallelogram, DL ? AB. If AB = 20 cm, AD = 13 cm and area of 
 
 
 
 
 
  
 
 
the parallelogram is 100 c m
2
, find AL. 
 
Solution: 
From the figure we have ABCD is a parallelogram with base AB = 20 cm and 
corresponding altitude DL. 
It is given that the area of the parallelogram ABCD = 100 c m
2
 
We know that the area of a parallelogram = Base x Height 
Therefore, 
100 = AB x DL 
100 = 20 x DL 
DL = 100/20 = 5 cm 
By observing the picture it is clear that we have to apply the Pythagoras theorem, 
Therefore by Pythagoras theorem, we have, 
(AD)
2 
= (AL)
2
 + (DL)
2
 
(13)
2
 = (AL)
2
+ (5)
2
 
(AL)
2
 = (13)
2
 – (5)
2
 
(AL)
2
 = 169 – 25  
= 144 
We know that 12
2 
= 144 
(AL)
2
 = (12)
2
 
AL = 12 cm 
Hence, length of AL is 12 cm. 
 
8. In Fig. 21, if AB = 35 cm, AD= 20 cm and area of the parallelogram is 560 cm
2
, find 
LB. 
 
 
 
 
 
  
 
 
 
Solution: 
From the figure, ABCD is a parallelogram with base AB = 35 cm and corresponding 
altitude DL. 
The adjacent side of the parallelogram AD = 20 cm. 
It is given that the area of the parallelogram ABCD = 560 cm
2
 
Now, Area of the parallelogram = Base x Height 
560 cm
2
 = AB x DL  
560 cm
2
 = 35 cm x DL 
DL = 560/35  
= 16 cm 
Again by Pythagoras theorem, we have, (AD)
2
 = (AL)
2
 + (DL)
2
 
(20)
2
= (AL)
2
 + (16)
2
 
(AL)
2 
= (20)
2 
– (16)
2
 
= 400 – 256 
= 144 
(AL)
2
 = (12)
2
 
AL = 12 cm 
From the figure, AB = AL + LB  
35 = 12 + LB 
LB = 35 – 12 = 23 cm 
Hence, length of LB is 23 cm. 
 
9. The adjacent sides of a parallelogram are 10 m and 8 m. If the distance between the 
longer sides is 4 m, find the distance between the shorter sides 
 
Solution: 
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