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 each side of a square is increased by 25%, find the percentage change in its area?

A) 65.25 B) 56.25
C) 65 D) 56
 
Answer & Explanation Answer: B) 56.25

Explanation:

let each side of the square be a , then area = a2 

As given that The side is increased by 25%, then 

New side = 125a/100 = 5a/4 

 

New area = 5a42  

 

Increased area= 25a216-a2 

 

Increase %=9a2/16a2*100  % = 56.25%

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437 90645
Q:

If the radius of a circle is decreased by 50%, find the percentage decrease in its area.

A) 55% B) 65%
C) 75% D) 85%
 
Answer & Explanation Answer: C) 75%

Explanation:

Let original radius = R. 

 

New radius = 50100R50100R 

 

Original area =R2  and new area = πR2

 

3πR24*1πR2*100 

 

Decrease in area = πR22=πR24 = 75%

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91 40395
Q:

If the diagonal of a rectangle is 17cm long and its perimeter is 46 cm. Find the area of the rectangle.

A) 110 B) 120
C) 130 D) 140
 
Answer & Explanation Answer: B) 120

Explanation:

let length = x and breadth = y then  

2(x+y) = 46         =>   x+y = 23  

x²+y² = 17² = 289  

now (x+y)² = 23² 

=> x²+y²+2xy= 529 

=> 289+ 2xy = 529

=> xy = 120  

area = xy = 120 sq.cm

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115 38373
Q:

A parallelogram has sides 30m and 14m and one of its diagonals is 40m long. Then its area is

A) 136 B) 236
C) 336 D) 436
 
Answer & Explanation Answer: C) 336

Explanation:

let ABCD be the given parallelogram 

area of parallelogram ABCD = 2 x (area of triangle ABC) 

now a = 30m, b = 14m and c = 40m

 s=1/2 x (30+14+40) = 42 

 

Area of triangle ABC = ss-as-bs-c  

 

= 4212282= 168sq m 

area of parallelogram ABCD = 2 x 168 = 336 sq m

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68 32202
Q:

A wire can be bent in the form of a circle of radius 56cm. If it is bent in the form of a square, then its area will be

A) 7744 B) 8844
C) 5544 D) 4444
 
Answer & Explanation Answer: A) 7744

Explanation:

length of wire = 2πr= 2 x (22/7 ) x 56 = 352 cm
side of the square = 352/4 = 88cm
area of the square = 88 x 88 = 7744sq cm

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60 31202
Q:

A man walked diagonally across a square lot. Approximately, what was the percent saved by not walking along the edges?

A) 30 B) 40
C) 50 D) 60
 
Answer & Explanation Answer: A) 30

Explanation:

Let the side of the square(ABCD) be x meters.

 

Then, AB + BC = 2x metres.

 

AC = 2x = (1.41x) m.

 

Saving on 2x metres = (0.59x) m.

 

Saving % =0.59x2x*100 = 30% (approx)

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51 30795
Q:

The area of the largest circle that can be drawn inside a rectangle with sides 18cm by 14cm is

A) 49 B) 154
C) 378 D) 1078
 
Answer & Explanation Answer: B) 154

Explanation:

The diameter is equal to the shortest side of the rectangle.

 So radius= 14/2 = 7cm. 

Therefore,  area of circle =πr2=227×49=154cm2

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45 28974
Q:

The length of a rectangular hall is 5m more than its breadth. The area of the hall is 750 sq.m. The length of the hall is

A) 20 B) 25
C) 30 D) 35
 
Answer & Explanation Answer: C) 30

Explanation:

Let breadth = x m

 

Then, length = (x+5)m

 

Area of a rectangle = Length x Breadth

 

x(x+5) = 750 

 

x² + 5x - 750= 0

 

(x+30)(x-25)= 0

 

x = 25 or x = -30

 

Hence, breadth x = 25m

 

=> Length = x + 5 = 25 + 5 = 30m. 

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50 28087