Shortcut Trick
For an equilateral triangle of side a, the circumradius (R) is given by the direct formula:
R = a ÷ √3
Given, side a = 12 cm.
R = 12 ÷ √3 = (4 × 3) ÷ √3 = 4√3 cm.
∴ The correct answer is 4√3 cm.
Alternate Method
Given:
Side of the equilateral triangle (a) = 12 cm
Formula Used:
Circumradius of an equilateral triangle (R) = a ÷ √3
Area of triangle (A) = (√3 ÷ 4) × a2
Circumradius (R) = (a × b × c) ÷ (4 × Area)
Calculations:
Using the standard circumradius formula for any triangle:
⇒ Area of the equilateral triangle = (√3 ÷ 4) × 122
⇒ Area = (√3 ÷ 4) × 144 = 36√3 cm2
⇒ Circumradius (R) = (12 × 12 × 12) ÷ (4 × 36√3)
⇒ R = 1728 ÷ 144√3
⇒ R = 12 ÷ √3
⇒ R = (12 × √3) ÷ (√3 × √3)
⇒ R = 12√3 ÷ 3 = 4√3 cm
Option Analysis:
Option (1) 8√3 cm: This is incorrect as it is double the actual circumradius (equal to the diameter of the circumcircle).
Option (2) 2√3 cm: This is incorrect as it represents the inradius (r = a ÷ 2√3 = 12 ÷ 2√3 = 2√3 cm) of the triangle, not the circumradius.
Option (3) 6√3 cm: This is incorrect as it represents the height of the equilateral triangle (h = (√3 ÷ 2) × a = 6√3 cm), not the circumradius.
Option (4) 4√3 cm: This is the correct circumradius calculated using the formula R = a ÷ √3.
∴ The correct answer is 4√3 cm.
Additional Information
Inradius of an Equilateral Triangle
The inradius (r) is the radius of the incircle and is given by the formula: r = a ÷ 2√3. For a side of 12 cm, r = 2√3 cm.
Relation between Circumradius and Inradius
For any equilateral triangle, the circumradius (R) is always twice the inradius (r), i.e., R = 2 × r.
Height of an Equilateral Triangle
The height (h) of an equilateral triangle is given by h = (√3 ÷ 2) × a. It is also the sum of the circumradius and the inradius (h = R + r).