ABC is a right-angled triangle. If the lengths of the two sides forming the right angle are 4 cm and 3 cm, what will be the radius of its incircle?
- AOption (a)
1 cm
- BOption (b)
4 cm
- COption (c)
2 cm
- DOption (d)
3 cm
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Correct answer
1 cm
Why this answer is correct
Shortcut Trick
For any right-angled triangle with sides forming the right angle as a and b, and hypotenuse h, the inradius (r) is given by:
r = (a + b − h) ÷ 2
Given: a = 3 cm, b = 4 cm. Hypotenuse h = √(32 + 42) = 5 cm (Pythagorean Triplet: 3, 4, 5).
r = (3 + 4 − 5) ÷ 2 = 2 ÷ 2 = 1 cm.
∴ The correct answer is 1 cm.
Alternate Method
Given:
Lengths of sides forming the right angle are 3 cm and 4 cm.
Formula Used:
- Pythagoras Theorem: Hypotenuse2 = Base2 + Perpendicular2
- Inradius (r): r = Area ÷ Semi-perimeter (s)
Calculations:
⇒ Hypotenuse (AC) = √(32 + 42) = √(9 + 16) = √25 = 5 cm
⇒ Area of triangle (A) = 1/2 × Base × Height = 1/2 × 4 × 3 = 6 cm2
⇒ Semi-perimeter (s) = (Side1 + Side2 + Side3) ÷ 2 = (3 + 4 + 5) ÷ 2 = 12 ÷ 2 = 6 cm
⇒ Inradius (r) = Area ÷ s = 6 ÷ 6 = 1 cm
∴ The correct answer is 1 cm.
Additional Information
Circumradius of Right Triangle
The radius of the circumcircle (R) of a right-angled triangle is always half of its hypotenuse (R = h ÷ 2).
General Inradius Formula
For any triangle, the inradius r = Δ ÷ s, where Δ is the area and s is the semi-perimeter.
Relation between Inradius and Circumradius
For a right-angled triangle, the relationship is 2R + 2r = a + b, where a and b are the perpendicular sides.





















