Three identical circles of radius cm touch each other externally. A smaller circle is placed in the central gap so that it touches all three circles externally. What is its radius, in cm?
A
Step-by-Step Solution
Key idea: inner circle in the symmetric gap between three equal touching circles.
Step 1: Centres of the three given circles form an equilateral triangle of side
Step 2: The gap circle centre is the triangle centre. Distance from triangle centre to a vertex is the circumradius:
Step 3: If the small radius is , external touch gives
Step 4: Solve:
Answer: Option A.
Trap: using subtraction here belongs to the outer enclosing circle case. The inner gap circle touches externally, so radii add.