Minimum Possible Distance

Geometry Level 2

ABC is an equilateral triangle of side 3 \sqrt3 m. P is an external point equidistant from the points A and C, such that:

PA + PB + PC = 3 \sqrt3 ( 3 + 2 \sqrt3 + 2 ) m

What is the minimum possible distance one has to cover from P, if one wants to be equidistant from the 3 vertices of A B C \triangle ABC ?

2 4 3 \sqrt3 3 2 \frac{\sqrt3}{2}

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1 solution

As A B C \triangle ABC is equilateral, we have:

BD = 3 2 \frac{\sqrt3}{2} x 3 \sqrt3 = 3 2 \frac{3}{2} m

Since P is equidistant from A and C, it will lie on the perpendicular bisector of AC.

Let PA = PC = a

Also, PB = PD + DB

\implies PB = ( a 2 ( 3 2 ) 2 \sqrt(a^{2} - (\frac{\sqrt3}{2})^{2} ) + 3 2 \frac{3}{2} = ( a 2 3 4 ) \sqrt(a^{2} - \frac{3}{4}) + 3 2 \frac{3}{2}

PA + PB + PC = 3 \sqrt3 ( 3 + 2 \sqrt3 + 2 )

\implies a + ( a 2 3 4 ) \sqrt(a^{2} - \frac{3}{4}) + 3 2 \frac{3}{2} + a = 3 \sqrt3 ( 3 + 2 \sqrt3 + 2 )

Solving for a, we get:

a = 3 \sqrt3 or a = 2 + 5 3 \frac{5}{\sqrt3}

From G (centroid as well as circumcenter of A B C \triangle ABC ), the distances from the vertices of the equilateral triangle will be the same. Also, G divides BD in the ratio 2:1.

Since PG should be the minimum possible, a must be equal to 3 \sqrt3 .

PG = DG + PD

= [ 1 3 \frac{1}{3} x 3 2 \frac{3}{2} ] + ( 4 a 2 3 ) 2 \frac{\sqrt(4a^{2} - 3)}{2}

Putting a = 3 \sqrt3 , we get:

= 1 2 \frac{1}{2} + ( 12 3 ) 2 \frac{\sqrt(12 - 3)}{2} = 2 m

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