Equilateral triangle (side 24 cm) with an inscribed circle:\nFind the area of the triangle’s region that is outside the circle but inside the triangle (in cm²).

Difficulty: Medium

Correct Answer: 98.5 sq. cm

Explanation:


Introduction / Context:
The “remaining portion” means triangle area minus the area of the incircle. For an equilateral triangle, the inradius has a simple formula in terms of side length, which makes computation straightforward and exact in radicals; the options here are rounded decimal values.


Given Data / Assumptions:

  • Equilateral triangle side a = 24 cm.
  • Inradius r = a√3/6.
  • Triangle area = (√3/4)a^2.


Concept / Approach:
Compute the triangle’s area and the incircle’s area, then subtract. Use either π = 22/7 or π ≈ 3.1416 for the circle; the rounded answer must match given options. Using standard exact expressions first helps consistency.


Step-by-Step Solution:

Triangle area = (√3/4)*24^2 = (√3/4)*576 = 144√3 ≈ 249.41 cm².Inradius r = 24√3/6 = 4√3 cm.Circle area = πr^2 = π*(4√3)^2 = π*48 ≈ 150.80 cm².Remaining area ≈ 249.41 − 150.80 ≈ 98.61 cm².


Verification / Alternative check:
Using π = 22/7 gives 48*(22/7) ≈ 150.86, still leading to ~98.55 cm². The closest option is 98.5 sq. cm.


Why Other Options Are Wrong:
36.6, 54.2, 72.8 are far from the precise subtraction.


Common Pitfalls:
Mistaking circumradius for inradius; forgetting that incircle area uses r, not R.


Final Answer:
98.5 sq. cm

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