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Question

Consider an equilateral triangle having vertices at the points A(23eiπ/2),B(23eiπ/6),C(23ei5π/6) Let P be any point on its incircle. then AP2+BP2+CP2=

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Solution

Let,
z1=23eiπ/2,z2=23eiπ/6,z3=23ei5π/6
Clearly, the points lie on the circle |z|=23 and ABC is equilateral triangle.
Here, z1+z2+z3=0¯¯¯¯¯z1+¯¯¯¯¯z2+¯¯¯¯¯z3=0
Since triangle is equilateral,
inradius r=OD=13 unit
Let P(z) be any point on the incircle, Now,
AP2=|zz1|2=|z|2+|z1|2(z¯¯¯¯¯z1+¯¯¯zz1)
Similarly,
BP2=|z|2+|z2|2(z¯¯¯¯¯z2+¯¯¯zz2)
CP2=|z|2+|z3|2(z¯¯¯¯¯z3+¯¯¯zz3)

AP2+BP2+CP2=3|z|2+|z1|2+|z2|2+|z3|2 z(¯¯¯¯¯z1+¯¯¯¯¯z2+¯¯¯¯¯z3)¯¯¯z(z1+z2+z3)
=5

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