Shortcut Trick
For a right-angled triangle with perpendicular P, base B, and hypotenuse H, the in-radius r is given by the direct formula:
r = (P + B − H) ÷ 2
Here, P = 6 cm, B = 8 cm, and hypotenuse H = √(62 + 82) = 10 cm.
⇒ r = (6 + 8 − 10) ÷ 2 = 2 cm
Perimeter of the in-circle = 2 × π × r = 2 × π × 2 = 4π cm.
∴ The correct answer is 4π cm.
Alternate Method
Given:
Right-angled triangle ABC with ∠A = 90°
Side AB = 6 cm
Side AC = 8 cm
Formula Used:
- Pythagoras Theorem: BC2 = AB2 + AC2
- In-radius of a triangle: r = Area ÷ Semi-perimeter (s)
- Perimeter of a circle: Circumference = 2 × π × r
Calculations:
First, find the hypotenuse BC using Pythagoras theorem:
⇒ BC = √(AB2 + AC2)
⇒ BC = √(62 + 82)
⇒ BC = √(36 + 64) = √100 = 10 cm
Now, calculate the area of the right-angled triangle ABC:
⇒ Area = (1 ÷ 2) × Base × Height
⇒ Area = (1 ÷ 2) × AC × AB
⇒ Area = (1 ÷ 2) × 8 × 6 = 24 cm2
Calculate the semi-perimeter (s) of the triangle:
⇒ s = (AB + BC + AC) ÷ 2
⇒ s = (6 + 10 + 8) ÷ 2 = 24 ÷ 2 = 12 cm
Calculate the in-radius (r) of the circle:
⇒ r = Area ÷ s
⇒ r = 24 ÷ 12 = 2 cm
Calculate the perimeter (circumference) of the in-circle:
⇒ Perimeter = 2 × π × r
⇒ Perimeter = 2 × π × 2 = 4π cm
Evaluation of Options:
⇒ Since the in-radius is exactly 2 cm, the perimeter must be 4π cm.
⇒ Option (1) 4.5π is incorrect because it corresponds to an in-radius of 2.25 cm.
⇒ Option (2) 3π is incorrect because it corresponds to an in-radius of 1.5 cm.
⇒ Option (3) 3.5π is incorrect because it corresponds to an in-radius of 1.75 cm.
⇒ Option (4) 4π is the only correct option as it matches the calculated perimeter of 4π cm.
∴ The correct answer is 4π cm.
Additional Information
In-radius of a Right-Angled Triangle
The in-radius r can be directly calculated using the formula r = (P + B − H) ÷ 2, where P is perpendicular, B is base, and H is hypotenuse.
Circumradius of a Right-Angled Triangle
The circumradius R of a right-angled triangle is exactly half of its hypotenuse, i.e., R = H ÷ 2. The circumcentre lies at the midpoint of the hypotenuse.
Relation between In-radius, Circumradius, and Sides
For any right-angled triangle, the relation between in-radius r, circumradius R, and the legs a and b is given by 2R + r = a + b.