FACTS  AND  FORMULAE  FOR  AREA  QUESTIONS

 

 

FUNDAMENTAL CONCEPTS :

I. Results on Triangles:

1. Sum of the angles of  a triangle is 180o

2. The sum of any two sides of a triangle is greater than the third side.

3. Pythagoras Theorem : In a right - angled triangle,

Hypotenuse2=Base2+Height2

4. The line joining the mid-point of a side of a triangle to the opposite vertex is called the median.

5. The point where the three medians of a triangle meet, is called Centroid. The centroid divides each of the medians in the ratio 2 : 1.

6. In an Isosceles triangle, the altitude from the vertex bisects the base.

7. The median of a triangle divides it into two triangles of the same area.

8. The area of the triangle formed by joining the mid-points of the sides of a given triangle is one-fourth of the area of the given triangle.

 

II.Results on Quadrilaterals :


1. The diagonals of a parallelogram bisect each other

2. Each diagonal of a parallelogram divides it into two triangles of the same area.

3. The diagonals of a rectangle are equal and bisect each other.

4. The diagonals of a square are equal and bisect each other at right angles

5. The diagonals of a rhombus are unequal and bisect each other at right angles

6. A parallelogram and a rectangle on the same base and between the same parallels are equal in area.

7. Of all the parallelogram of given sides, the parallelogram which is a rectangle has the greatest area.

 

IMPORTANT FORMULAE

I. 

1. Area of a rectangle = (length x Breadth)

Length =AreaBreadth  and  Breadth=AreaLength

2. Perimeter of a rectangle = 2( length + Breadth)

 

 

II. Area of square = side2=12diagonal2 

 

III. Area of 4 walls of a room = 2(Length + Breadth) x Height

 

 

IV.

1. Area of a triangle =12×base×height

2. Area of a triangle = s(s-a)(s-b)(s-c), where a, b, c are the sides of the triangle and s=12a+b+c

3. Area of an equilateral triangle =34×side2

4. Radius of incircle of an equilateral triangle of side a=a23

5. Radius of circumcircle of an equilateral triangle of side a=a3

6. Radius of incircle of a triangle of area  and semi-perimeter s=s

 

 

V.

1. Area of a parallelogram = (Base x Height)

2. Area of a rhombus = 12×Product of diagonals

3. Area of a trapezium = 12×(sum of parallel sides)×distance between them

    

 

VI.

1. Area of a cicle = πR2, where R is the radius.

2. Circumference of a circle = 2πR.

3. Length of an arc = 2πRθ360, where θ is the central angle.

4. Area of a sector = 12arc×R=πR2θ360 

 

VII.

1. Area of a semi-circle = πR22

2. Circumference of a semi - circle = πR

Q:

If the area of a square with side a s equal to the area of a triangle with base a, then the altitude of the triangle is

A) a B) 2a
C) 5a D) 3a
 
Answer & Explanation Answer: B) 2a

Explanation:

Area of a square with side a = a² sq unts

Area of a triangle with base a = (1/2) * a * h sq.unts

a² =1/2 *a *h

=> h = 2a

altitude of the triangle is 2a

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1 5707
Q:

Calculate the area (in cm2) of a circle whose diameter is 7 cm.

 

A) 38.5   B)  77    
C) 32.5   D) 65
 
Answer & Explanation Answer: A) 38.5  

Explanation:
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Q:

A circular piece of thin wire is converted into a rhombus of side 11 cm. Find the diameter of the circular piece?

A) 28 cm B) 3.5 cm
C) 7 cm D) 14 cm
 
Answer & Explanation Answer: D) 14 cm

Explanation:

Circular piece is 4 x 11 = 44 cm long,

Then Circumference of circle is given by,

44 = pi x D, where D is the diameter

D = 44 / pi

Take pi = 22 / 7, then

D = 44 / (22/7) = (44 x 7) / 22

D = 14 cm.

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24 5658
Q:

Angles of the triangle are in the ratio of 1:2:3. Choose the correct triangle for the given ratio.

 

A) Equilateral B) Isosceles
C) Right angle D) Obtuse angled
 
Answer & Explanation Answer: C) Right angle

Explanation:
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Q:

Area of a square and a rectangle are equal. If side of the square is 40 cm and length of the rectangle is 64 cm, what is the perimeter of the rectangle ?

A) 187 cm B) 178 cm
C) 149 cm D) 194 cm
 
Answer & Explanation Answer: B) 178 cm

Explanation:

Area of square = 40 x 40

= 1600 sq.cm

Given that the areas of Square and Rectangle are equal

=> Area of rectangle = 1600 Sq.cm

We know that, Area of rectangle = L x B

Given L = 64 cm

Breadth of rectangle = 1600/64 = 25 cm

Perimeter of the rectangle = 2(L + B) = 2(64+25) = 178 cm.

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32 5474
Q:

Which of the following conditions is correct for a right angled triangle?

 

A) Sum of the two angles of a triangle is greater than the third angle B) Sum of the two angles of a triangle is less than the third angle
C) Sum of the two angles of a triangle is equal to the third angle D) None of these
 
Answer & Explanation Answer: C) Sum of the two angles of a triangle is equal to the third angle

Explanation:
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Q:

The area of a square is 30.25 cm². Find its perimeter (in cm).

 

A) 44 B) 23
C) 22 D) 46
 
Answer & Explanation Answer: C) 22

Explanation:
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Q:

Which of the following combination of sides results in the formation of obtuse angled triangle?

 

A) 6,7,13   B)  5, 6, 8  
C)  4, 5, 6   D)  None of these
 
Answer & Explanation Answer: B)  5, 6, 8  

Explanation:
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