NCERT Solutions
Class 9 Maths
Heron’s Formula

Ex.12.2 Q.3
Radha made a picture of an aeroplane with coloured paper as shown in the figure.
Find the total area of the paper used.
For the triangle marked I:
a = 5 cm, b = 5 cm, c = 1 cm
s = (a + b + c) = (5 + 5 + 1) ÷ 2 = 11 ÷ 2 = 5.5 cm
Now, Area of the triangle = √ {s (s - a) (s - b) (s - c)}
= √ {5.5(5.5 - 5) (5.5 - 5) (5.5 - 1)}
= √ {5.5 × 0.5 × 0.5 × 4.5}
= √6.1875
= 2.5 cm2 (approx.)
For the rectangle marked II:
Length = 6.5 cm, Breadth = 1 cm
Area of the rectangle = 6.5 * 1 cm2 = 6.5 cm2
For the trapezium marked III:
Draw AF || DC and AE ⊥ BC
AD = FC = 1 cm, DC = AF = 1 cm
So, BF = BC – FC = (2 – 1) cm = 1 cm
Hence, ∆ABF is equilateral.
Also, E is the mid-point of BF.
Now, BE = 1/2 cm = 0.5 cm
Also, AB2 = AE2 + BE2
[Pythagoras theorem]
=> AE2 = 12 – (0.5)2
=> AE2 = 1 – 0.25
=> AE2 = 0.75
=> AE = √0.75
⇒ AE = 0.9 cm (approx.)
Area of the trapezium = × (sum of the parallel sides) × (distance between) them.
= × (BC + AD) × AE
= × (2 + 1) × 0.9
= 12 × 3 × 0.9
= 2.7 ÷ 2
= 1.4 cm2
For the triangle marked IV:
It is a right-triangle
∴ Area of the triangle = × base × height
= × 6 × 1.5
= 3 × 1.5
= 4.5 cm2
For the triangle marked V:
This triangle is congruent to the triangle marked IV.
Hence, area of the triangle = 4.5 cm2
Total area of the paper used = (2.5 + 6.5 + 1.4 + 4.5 + 4.5) cm2
= 19.4 cm2