ΔABC is inscribed in a circle with Centre O. If AB = 21 cm, BC = 20 cm and AC = 29 cm, then what is the length of the circumradius of the triangle?
- AOption (a)
13.5 cm
- BOption (b)
21.5 cm
- COption (c)
32.5 cm
- DOption (d)
14.5 cm
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Correct answer
14.5 cm
Why this answer is correct
Shortcut Trick
Check for a Pythagorean triplet: 202 + 212 = 400 + 441 = 841, and 292 = 841.
Since AB2 + BC2 = AC2, ΔABC is a right-angled triangle.
The circumcentre of a right-angled triangle always lies exactly at the midpoint of its hypotenuse.
⇒ Circumradius = Hypotenuse ÷ 2
⇒ Circumradius = 29 ÷ 2 = 14.5 cm
∴ The correct answer is 14.5 cm.
Alternate Method
Given:
In ΔABC:
⇒ AB = 21 cm, BC = 20 cm, and AC = 29 cm
Formula Used:
⇒ Circumradius (R) = (a × b × c) ÷ (4 × Area)
⇒ Semi-perimeter (s) = (a + b + c) ÷ 2
⇒ Area = √[s(s − a)(s − b)(s − c)]
A C B O 21 cm 20 cm 29 cm R
Calculations:
First, find the semi-perimeter (s) of the triangle:
⇒ s = (21 + 20 + 29) ÷ 2 = 70 ÷ 2 = 35 cm
Calculate the area using Heron's formula:
⇒ Area = √[35 × (35 − 21) × (35 − 20) × (35 − 29)]
⇒ Area = √[35 × 14 × 15 × 6]
⇒ Area = √44100 = 210 cm2
Substitute the values into the circumradius formula:
⇒ R = (21 × 20 × 29) ÷ (4 × 210)
⇒ R = 12180 ÷ 840
⇒ R = 14.5 cm
∴ The correct answer is 14.5 cm.
Additional Information
Circumcentre of a Triangle
It is the point of intersection of the perpendicular bisectors of the sides. The distance from the circumcentre to all vertices is equal to the circumradius (R).
Location of Circumcentre
For an acute triangle, it lies inside. For an obtuse triangle, it lies outside. For a right-angled triangle, it lies exactly at the midpoint of the hypotenuse.
Inradius of a Right Triangle
The inradius (r) can be directly found using the formula: r = (a + b − c) ÷ 2, where a and b are perpendicular sides and c is the hypotenuse.




























