Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9

Class 9: Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9

The document Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9 is a part of the Class 9 Course Mathematics (Maths) Class 9.
All you need of Class 9 at this link: Class 9

Q1. A traffic signal board, indicating ‘School Ahead’, is an equilateral triangle with side ‘a’. Find the area of the signal board using Heron’s formula. If its perimeter is 180 cm, what will be the area of the signal board?
Ans: For an equilateral triangle with side ‘a’, area = (√3/4) a2 

Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9

∵ Each side of the triangle = a cm
∴ a + a + a = 180 cm
3a = 180 cm
a= (180/3) = 60 cm
Now, s = Semi-perimeter = (180/2) = 90 cm
∵ Area of triangle  Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
Area of the given triangle
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
= 30 x 30 x√3 = 900√3 cm2
Thus, the area of the given triangle = 900√3cm2.

Q2. The triangular side walls of a flyover have been used for advertisements. The sides of the walls are 122 m, 22 m, and 120 m (see fig). The advertisements yield an earning of Rs. 5000 per m2 per year. A company hired one of its walls for 3 months. How much rent did it pay?

Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9Ans: The sides of the triangular wall are a = 122 m, b = 120 m, c = 22 m.
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
∵ The area of a triangle is given by
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
∵ Rent for 1 year (i.e. 12 months) per m2 = Rs. 5000
∴ Rent for 3 months per m2 = Rs. 5000 x (3/12)
⇒ Rent for 3 months for 1320 m2 = 5000 x (3/12) x 1320 = 5000 x 3 x 110 = Rs.16,50,000.

Q3. There is a slide in a park. One of its side walls has been painted in some colour with a message “ KEEP THE PARK GREEN AND CLEAN” (See figure). If the sides of the wall are 15 m, 11 m and 6 m, find the area painted in colour.

Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9

Ans: Sides of the wall a = 15 m, b = 11 m, c = 6 m.
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
 The area of the triangular surface of the wall
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
Thus, the required area painted in colour = 20√2 m2.

Q4. Find the area of a triangle, two sides of which are 18 cm and 10 cm and the perimeter is 42 cm.
Ans: Let the sides of the triangle be a = 18 cm, b = 10 cm and c =?
∵ Perimeter (2s) = 42 cm
s = (42/2)  = 21 cm
c = 42 – (18 + 10) cm = 14 cm
∵ Area of a triangle = Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
∴ Area of the given triangle
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
Thus, the required area of the triangle = 21√11 cm2.

Q5. Side of a triangle is in the ratio of 12: 17: 25 and its perimeter is 540 cm. Find its area.
Ans: The perimeter of the triangle = 540 cm.
Semi-perimeter of the triangle, s= (540/2) = 270 cm
∵ The sides are in the ratio of 12: 17: 25.
∴ Let, the side be a = 12x cm, b = 17x cm, c = 25x cm.
12x + 17x + 25x = 540
54x = 540
x = (540/54) = 10
∴ a = 12 x 10 = 120 cm, b = 17 x 10 = 170 cm, c = 25 x 10 = 250 cm.
(s – a) = (270 – 120) cm = 150 cm
(s – b) = (270 – 170) cm = 100 cm
(s – c) = (270 – 250) cm = 20 cm
∴ Area of the triangle
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
= 10 x 10 x 3 x 3 x 5 x 2 cm2 = 9,000 cm2
Thus, the required area of the triangle = 9,000 cm2.

Q6. An isosceles triangle has a perimeter of 30 cm and each of the equal sides is 12 cm. Find the area of the triangle.
Ans: Equal sides of the triangle are 12 cm each.
Let the third side = x cm.

Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9∵ Perimeter = 30 cm
∴ 12 cm + 12 cm + x cm = 30 cm
x = 30 – 12 – 12 = 6 cm
Semi-perimeter = (30/2) cm = 15 cm
∴ Area of the triangle
Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9
Thus, the required area of the triangle = 9√15 cm2.

The document Ex 12.1 NCERT Solutions- Heron’s Formula Notes | Study Mathematics (Maths) Class 9 - Class 9 is a part of the Class 9 Course Mathematics (Maths) Class 9.
All you need of Class 9 at this link: Class 9

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