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Install NowNCERT Solutions for Class 9 Maths Exercise 12.1 book solutions are available in PDF format for free download. These ncert book chapter wise questions and answers are very helpful for CBSE board exam. CBSE recommends NCERT books and most of the questions in CBSE exam are asked from NCERT text books. Class 9 Maths chapter wise NCERT solution for Maths Book for all the chapters can be downloaded from our website and myCBSEguide mobile app for free.
NCERT solutions for Class 9 Maths Herons Formula Download as PDF
NCERT Solutions for Class 9 Mathematics Herons Formula
1. A traffic signal board, indicating ‘SCHOOLAHEAD’ is an equilateral triangle with side 
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. Let the Traffic signal board is 
ABC.
According to question, Semi-perimeter of ABC 
= 
Using Heron’s Formula, Area of triangle ABC = 
= 
= 
= 
= 
Now, Perimeter of this triangle = 180 cm 
Side of triangle 
= 
= 60 cm
Semi-perimeter of this triangle = 
= 90 cm
Using Heron’s Formula, Area of this triangle =
= 
= 
= 
= 
2. The triangular side walls of a flyover has been used for advertisements. The sides of the walls are 122 m, 22 m and 120 m (see figure). The advertisement 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?
Ans. Given: 
= 122 m, 
= 22 m and 
= 120 m
Semi-perimeter of triangle = 
= 
= 132 m
Using Heron’s Formula,
Area of triangle =
= 
= 
= 
Rent for advertisement on wall for 1 year = Rs. 5000 per 
Rent for advertisement on wall for 3 months for 1320 m2 = 
= Rs. 1650000
Hence rent paid by company = Rs. 16,50,000
3. There is 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.
Ans. Since, sides of coloured triangular wall are 15 m, 11 m and 6 m.
Semi-perimeter of coloured triangular wall = 
= 
= 16 m
Now, Using Heron’s formula,
Area of coloured triangular wall =
= 
= 
= 
Hence area painted in blue colour =
4. Find the area of a triangle two sides of which are 18 cm and 10 cm and the perimeter is 42 cm.
Ans. Given: = 18 cm, = 10 cm.
Since Perimeter = 42 cm
=42
18 + 10 + = 42
= 42 – 28 = 14 cm
Semi-perimeter of triangle = 
= 21 cm
Area of triangle =
= 
= 
= 
= 
= 21 x 3.3 = 69.3 cm2
NCERT Solutions for Class 9 Maths Exercise 12.1
5. Sides of a triangle are in the ratio of 12: 17: 25 and its perimeter is 540 cm. Find its area.
Ans. Let the sides of the triangle be 
and 
.
Therefore, 
= 540
= 10
The sides are 120 cm, 170 cm and 250 cm.
Semi-perimeter of triangle 
= 270 cm
Now, Area of triangle =
= 
= 
= 
NCERT Solutions for Class 9 Maths Exercise 12.1
6. An isosceles triangle has perimeter 30 cm and each of the equal sides is 12 cm. Find the area of the triangle.
Ans. Given: = 12 cm, = 12 cm
Since Perimeter = 30 cm
=30
12 + 12 + = 30
= 30 – 24 = 6 cm
Semi-perimeter of triangle = 
= 15 cm
Area of triangle =
= 
= 
= 
= 
NCERT Solutions for Class 9 Maths Exercise 12.1
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