UPSC CDS 1 2020 Math Paper

The centre of three lower balls form an equilateral triangle. Let C’ be the centroid of this triangle. C’ is at the distance of

2r

√3

form, each vertex of lower ball.
Let h = height of centre of 4th ball from C’.=√(2r)2−(

2r

√3

)2=2r√

2

3

∴ height of centre of 4th ball from plane