Class 9 Mathematics

Chapter 6 Solutions — Measuring Space: Perimeter and Area

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Step-by-step NCERT solutions for Measuring Space: Perimeter and Area (Chapter 6, NCERT Class 9 Mathematics) — the full working for every question, not just the final answer. You can also read the Measuring Space: Perimeter and Area textbook chapter.

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All 56 questions in Measuring Space: Perimeter and Area are solved in the PDF. Here's what's inside, exercise by exercise:

Perimeter of a Circle and Arc Length

The perimeter of a circle is 44 cm. What is its radius?
2.
Calculate, correct to 3 significant figures, the circumference of a circle with:
- (i) radius 7 cm,
- (ii) radius 10 cm,
- (iii) radius 12 cm.
3.
Calculate the length of the arc of a circle if:
- (i) the radius is 3.5 cm and the angle at the centre is 60°, and
- (ii) the radius is 6.3 m and the angle at the centre is 120°.
Find the perimeter of a sector (i.e., the curved portion as well as the two straight portions) of a circle of radius 14 cm and sector angle 75°.
Find the perimeters of the following shapes (taking the arcs to be quarter or half or three-quarters of a circle, as appropriate) (Fig. 6.14i to 6.14ix).
6.
If the diameter of a car tyre is 56 cm, then:
- (i) How far does the car need to travel for the tyre to complete one revolution?
- (ii) How many revolutions does the tyre make if the car travels 10 km?
7.
Find the total perimeter of all the petals in each of the given flowers.
- (i) Fig. 6.15A: 4-petal flower, centres of arcs at midpoints of sides of a square of side 14 cm.
- (ii) Fig. 6.15B: 6-petal flower, centres of arcs at vertices of a regular hexagon of side 42 cm.
The ratio of the perimeters of two circles is 5:4. What is the ratio of their radii?

Area of Triangles, Parallelograms, and Quadrilaterals

Find the area of triangle ADE in Fig. 6.31, where ABCD is a rectangle with DC = 10 cm, and E is a point on BC such that EC = 8 cm; A is at the same height as E (so AE is horizontal and AD is vertical).
The parallel sides of a trapezium are 40 cm and 20 cm. If its non-parallel sides are both equal, each being 26 cm, find the area of the trapezium.
Find the area of a triangle, given that its sides are 8 cm and 11 cm long, and its perimeter is 32 cm.
The sides of a triangular plot are in the ratio 3:5:7; its perimeter is 300 m. Find its area.
One diagonal of a rhombus is twice as long as the other diagonal. If the rhombus has area 128 cm², find the length of the shorter diagonal.
ABCD is a parallelogram. P and Q are any two points on side AB. What can you say about the ratio area(△PCD) : area(△QCD)?
O is any point on the diagonal PR of a parallelogram PQRS. Prove that the areas of triangles PSO and PQO are equal.
If the mid-points of the sides of a 4-gon are joined in order, prove that the area of the parallelogram thus formed will be half of the area of the given 4-gon.
In △ABC, the midpoint of BC is D (Fig. 6.32). Median AD is drawn. P is any point on AD. Show that area(△ABP) = area(△ACP).
Given a square ABCD, let P be a point within it. Join PA, PB, PC, PD (Fig. 6.33). What is the ratio of the areas of the red region (△PAB and △PCD) and the green region (△PBC and △PDA)?
In △ABC, D is the midpoint of AB. P is any point on BC, and Q is a point on AB such that CQ ∥ PD. PQ is joined (Fig. 6.34). Prove that Area(△BPQ) = (1/2) × Area(△ABC).

Area of a Circle, Sector, and Segment

Find the area of a sector of a circle with radius 7 cm if the angle of the sector is 60°.
Find the area of a quadrant of a circle whose circumference is 44 cm.
The length of the minute hand of a clock is 7 cm. Find the area swept by the minute hand in 10 minutes.
4.
A chord of a circle of radius 10 cm subtends 90° at the centre. Find the area of the corresponding:
- (i) minor sector (that subtends 90° at the centre), and
- (ii) major sector (that subtends 270° at the centre). (Use π ≈ 3.14.)
A chord of a circle of radius 15 cm subtends an angle of 60° at the centre of the circle. Find the areas of the corresponding minor and major segments of the circle. (Use π ≈ 3.14 and √3 ≈ 1.73.)
A car has two wipers which do not overlap. Each wiper has a blade of length 28 cm and sweeps through an angle of 120°. Find the total area cleaned at each sweep of the blades.
*A chord of a circle of radius r subtends an angle of 60° at the centre of the circle. Show that the area of the corresponding minor segment of the circle is equal to πr²(1/6 − √3/4).
*An equilateral triangle is inscribed in a circle of radius r. Show that the ratio of the area of the triangle to the area of the circle is equal to 3√3/(4π) ≈ 0.413.
*A square is inscribed in a circle of radius r. Show that the ratio of the area of the square to the area of the circle is equal to 2/π ≈ 0.637.
*A hexagon is inscribed in a circle of radius r. Show that the ratio of the area of the hexagon to the area of the circle is equal to 3√3/(2π) ≈ 0.827. Can you see why the answer is exactly twice the answer to Question 8?

Exercises

Identities in algebra can sometimes be shown as area relationships. The area model shown corresponds to the identity (a + b)² = a² + 2ab + b². Draw figures corresponding to the identities (a + b)(a – b) = a² – b² and (a + b + c)² = a² + b² + c² + 2ab + 2bc + 2ca.
An isosceles triangle has perimeter 40 cm; the equal sides are 15 cm each. Find the area of the triangle.
An isosceles triangle has base 10 cm, and its area is 60 cm². What are the lengths of the equal sides?
The area of a right-angled triangle is 54 sq. cm. One of its legs has length 12 cm. Find its perimeter.
The sides of a triangle are in the ratio 2 : 3 : 4, and its perimeter is 45 cm. Find its area.
The sides of a triangle have lengths 7 cm, 24 cm, 25 cm. Find the area of the triangle in two different ways.
If the wheel of a bicycle has a diameter of 60 cm, find how far a cyclist will have travelled after the wheel has rotated 100 times.
Find the area of a quadrant of a circle whose circumference is 66 cm.
The wheel of a car has an outer radius of 28 cm. Calculate how far the car travels after one complete turn of the wheel, and how many times the wheel turns during a journey of 1 km.
Two rectangles have the same area and the same perimeter. Does this mean that they are congruent to each other?
You know that the area of a parallelogram is base × height. Using this and the figure, show that the area of a trapezium is half the sum of the parallel sides × height, i.e., ½(a + b)h.
By dividing a trapezium into two triangles show that its area is half the sum of the parallel sides multiplied by the height (the same formula as the one given above).
Show how we can use two identical copies of a trapezium to make a parallelogram. How will this give us the formula for the area of a trapezium?
14.
Show that the area of a kite is half the product of its diagonals. Show this:
- (i) using algebra, and
- (ii) using geometry.
15.
Three problems about fitting congruent shapes together.
- (i) Rectangle ABCD has sides a, b, and rectangle PQRS has sides 2a, 2b. Show that PQRS has 4 times the area of ABCD. Does this mean that 4 copies of rectangle ABCD will fit into rectangle PQRS? Check and see!
- (ii) △ABC has sides a, b, c, and △PQR has sides 2a, 2b, 2c. Show that △PQR has 4 times the area of △ABC. Does this mean that 4 copies…
Fig. 6.43: What fraction of the triangle is shaded? Fig. 6.44: What fraction of the square is shaded?
Fig. 6.45: What fraction of the rectangle is covered by the circles? Fig. 6.46: What fraction of the rectangle is covered by the circles?
Use the above to make a conjecture about the area occupied by circles fitted into a rectangle in the manner shown. Test your conjecture for particular cases: 10 circles; 20 circles; 50 circles. Then prove your conjecture!
The figure (Fig. 6.47) shows nine identical rectangles fitted together to make a large rectangle whose area is 72 cm². Find the perimeter of each small rectangle.
Fig. 6.48 shows lines drawn from a vertex of a triangle to the points of trisection of the opposite side. Show that the areas of the shaded blue triangle and the shaded red triangle are equal.
Fig. 6.49 shows a quarter circle in a square with its centre at one vertex, passing through the two adjacent vertices. There are two semicircles on two adjacent sides as diameters. They create the shaded regions A and B. Show that A and B have equal area.
In Fig. 6.50, four semicircles have been drawn within the given square whose side is 2 units. The centres of these semicircles are the midpoints of the sides. They create a 4-petalled flower (shown in blue). Find the perimeter and the area of this flower.
In Fig. 6.51 we see two concentric circles with a common centre O. A chord BC of the larger circle is drawn, touching the smaller circle at A. The length of BC is l. Show that the area of the green region enclosed between the two circles is (1/4)πl².
In Fig. 6.52, semicircles have been drawn on all the sides of a right-angled triangle as shown. Show that Area(A) + Area(B) = Area(C).
Fig. 6.53 shows two congruent circles (radius r) passing through each other's centres. Find the area of the region enclosed by the two circles in terms of the common radius r.
In Fig. 6.54, we see three triangles within a rectangle. The areas of the triangles are A, B, C, as marked. Show that the area of the rectangle is 2(A + C)(B + C)/C.
In Fig. 6.55 we see two shaded regions formed by a quarter circle, a semicircle, and a triangle. Show that the areas of the two shaded regions are equal.

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