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Class 9 Maths notes for Heron Formula Mensuration



Mensuration

  • It is branch of mathematics which is concerned about the measurement of length ,area and Volume of plane and Solid figure

Perimeter

  • The perimeter of plane figure is defined as the length of the boundary
  • It units is same as that of length i.e. m ,cm,km

1 Meter

10 Decimeter

100 centimeter

1 Decimeter

10 centimeter

100 millimeter

1 Km

10 Hectometer

100 Decameter

1 Decameter

10 meter

1000 centimeter

Area

  • The area of the plane figure is the surface enclosed by its boundary
  • It unit is square of length unit. i.e. m2 ,  km2

1 square Meter

100 square Decimeter

10000 square centimeter

1 square Decimeter

100 square centimeter

10000 square millimeter

1 Hectare

100 squareDecameter

10000 square meter

1 square myraimeter

100 square kilometer

108  square meter

Perimeter and Area of Different Figure

 

N

Shape

Perimeter/height

Area

1

Right angle triangle

Base =b, Height =h

Hypotenuse=d

P=b+h+d

Height =h

A=(1/2)BH

2

Isosceles right angled triangle

Equal side =a

 

Height=a

A=(1/2)a1/2

 

3

Any triangle of sides a,b ,c

P=a+b+c

 

 

A=[s(s-a)(s-b)(s-c)]1/2

s=(a+b+c)/2

 

This is called   Heron's formula (sometimes called Hero's formula) is named after Hero of Alexandria

4

Square

Side =a

  1.  

A=a2

5

Rectangle of Length and breath L and B respectively

P=2L +2B

A=LX B

 

6

Parallelograms

Two sides are given as a and b

P=2a+2b

A= BaseX height

When the diagonal is also given ,say d

Then

A=[s(s-a)(s-b)(s-d)]1/2

s=(a+b+d)/2

 

 

7

Rhombus

Diagonal d1 and d2 are given

p=2(d12+d22)1/2

Each side=(1/2)(d12+d22)1/2

  

 

A=(1/2)d1d2

8

Quadrilateral

a) All the sides are given a,b,c ,d

b) Both the diagonal are perpendicular to each other

 

c) When a diagonal and perpendicular to diagonal are given

a) P=a+b+c+d

a)

A=[s(s-a)(s-b)(s-c)(s-d)]1/2

s=(a+b+c+d)/2

b)A=(1/2)d1d2

where d1 and d2  are the diagonal

c)A=(1/2)d(h1+h2)

where d is diagonal and h1 and h2 are perpendicular to that

How to solve the Area and Perimeter problems

1) We must remember the formula for all the common figures as given above the table

2) Find out what all is given in the problem

3) Convert all the given quantites in the same unit

4) Sometimes Perimeter is given and some side  is unknown,So you can calculate the sides using the Perimeter

5) If it is a complex figure ,break down into common know figures like square,rectangle,triangle

6) Sometimes we can find another side using pythogorus theorem in the complex  figure

7) If common figure, apply the formula given above and calculate the area.

8) If complex figure, calculate the area for each common figure in it  and sum all the area at the end to calculate the total area of the figure

Solved Examples

1) A right angle traingle has base 20 cm and height as 10 cm, What is the area of the traingle?

Solution

Given values B=20 cm

H=10 cm

Both are in same units

A=(1/2)BH=100 cm

2) Sides of traingles  are in the ratio  12:17:25.  The perimeter of the traingle is 540 cm. Find out the area of the traingle?

Solution

Let the common ration between the sides be y,then sides are 12y,17y,25 y

Now we know the perimeter of the triangle is given by

P=a+b+c

540=12y+17y+25y

or y=10 cm

Now Area of triangle is   =[s(s-a)(s-b)(s-c)]1/2

Where s=(a+b+c)/2

Here s=270 cm

a=120 cm

b=170cm

c=250cm

 

Substituting all these values in the area equation,we get

A=9000cm2

3) A equilateral triangle is having side 2 cm. What is the area of the triangle?

Solution:

We know that Are of equilateral triangle is given by

A=[(3)1/2 a2]/4

Substituting the values given above

A=(3)1/2

 

We can summarize various method to calculate the Area of the Triangle

If you know the altitude  and Base

Area =(1/2)BH
If you all the three sides

A=[s(s-a)(s-b)(s-c)]1/2

s=(a+b+c)/2

If it is isoceles traingle with equal side a A=(1/2)a1/2
If it is equilateral triangel  with equal side a A=[(3)1/2 a2]/4
If it is right angle triangle with Base B and Height H Area =(1/2)BH
   

 



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