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# TNPSC – MENSURATION

## TNPSC – MENSURATION

### MENSURATION:

This chapter is based finding on 2D & 3D figures and basically two-dimensional figures come under geometry. Only 3-dimensional figures are studied their area and volume.

#### Two Dimensional Figures:

=> Rectangle (opposite sides are equal and intersect each other at an angle of 90°)

Perimeter = 2 (Length * Breadth)

=> Square (All the four sides are equal and the angle enclosed by adjacent sides is 90°)

Area = (Side)2

Perimeter = 4*side

Diagonal = √2*side

After this all the four sides figures —

Parallelogram

Rhombus

Trapezium & Scalene Quadrilateral should be given.

=> Triangles

(i) If a,b and c are the lengths of the first, second and third sides of a triangle respectively, then

S = (a+b+c)/2, where s=semi-perimeter and Area = √[s (s-a) (s-b) (s-c)]

(Heron’s formula),

Perimeter = a+b+c

(ii) For a right angled triangle,

Area = (1/2) base * height, and

Perimeter = a+b+c

(iii) For an equilateral triangle

Area = √ (3/4) * (side)2 , and

Perimeter = 3*side

=> Circle

=> Area of the walls of a room

= 2 * height (length + breadth)

Height = Wall Area / 2 (length + breadth)

==> Parallelogram (Opposite sides are equal & parallel, but angel enclosed by adjacent sides is not a right angle)

Area = base * height

Height = length of perpendicular dropped from on opposite side to the base.

Perimeter = 2 (sum of opposite sides)

(ii) Rhombus (All the four sides are equal but the angle enclosed by adjacent sides is not a right angle)

Area = ½ * product of diagonals

Perimeter = 4 * side

(iii) Trapezium (One pair of opposite side is parallel)

Area = ½ * (Sum of parallel sides) * height

Perimeter = Sum of 4 sides, i.e. a+b+c+d.

(iv) Scalene Quadrilateral (All the four sides are unequal and non-parallel)

Area = ½ (DP+BQ) * AC

Perimeter = Sum of 4 sides, i.e. a+b+c+d.

Note:

Area is the portion enclosed by the figure.

Perimeter is the boundary sum of angles of a four sided figure is 360o.

#### Three dimensional figures

=> Cuboid

If L, B and H are length, breadth and height of the cuboid, then

Volume = L*B*H

Surface area = 2 (L*B+B*H+H*L)

Diagonal = √ (L2+B2+H2

=> Cube

If a is each side of the cube, then

Volume = a*a*a=a3

Surface area = 2(a*a+ a*a+ a*a) = 6a2

Diagonal of cube = √ (a2+a2+a2) = √ (3a)

=> Cylinder

If radius of cylinder is r and height or length is h, then

Volume = πr2h

Latest surface area = 2πrh

Whole surface area = (2πrh + 2πr2)

=> Cone

If base-radius, vertical height and slanting height of a cone are r, h and l respectively, then

Volume = 1/3 (πr2h)

Lateral surface area = πrl

Total surface area = πrl+ πr2

Vertical height = l = √ (r2+h2)

Frustum of a cone:

A cone whose top is sliced off, is called frustum of a cone.

In the above figure, R is the radius of the base, r is the radius of the top, h is the vertical height and L is the slant height.

Volume = πh/3 (R2+r2+Rr)

Slant height = √ [(R-r)2+(h2)]

Curved surface area = π(R+r)L

Total surface area = π (RL+rL+r2+R2)

To find height of the original cone, following formula can be used –

H = Rh/(R-r)

=>Sphere

If r is the radius of the sphere, then

Volume = 4/3 (πr3)

Surface area = 4πr2

=> Hemisphere

Volume = 2/3 (πr3)

Curved surface area = 2πr2

Total surface area = 2πr2+πr2 = 3πr2

Some units related to volume:

1 litre = 1000 cm3

1 Hectometer3 = 1000000 meter3

1 Decameter3 = 1000 meter3

1 Meter3 = 1000000 cm3

1 Decimeter3 = 1000cm3

1 Millimetre3 = 1/1000 cm3

Some units to related to area:

1 Hectare = 10000 metre square

1 kilometre square = 1000000 metre square

1 Decametre square = 100 metre square

1 Decimetre square = 1/100 metre square

1 Centimetre square = 1/10000 metre square

1 Millimetre square = 1/1000000 metre square

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