Question
Determine the points xy-plane equidistant from the points A(1, -1, 0), B(2, 1, 2) and C(3, 2, -1).

Answer

Let the point on xy-plane be P(x, y, 0).

Now P is equidistance from A(1, -1, 0), B(2, 1, 2) and C(3, 2, -1).

So, AP - BP - CP

Now,

(AP)2 = (x - 1)2 + (y + 1)2 + (0 - 0)2

(BP)2 = (x - 2)2 + (y - 1)2 + (0 - 2)2

(CP)2 = (x - 3)2 + (y - 2)2 + (0 + 1)2

(AP)2 = (BP)2 ⇒ (x - 1)2 + (y + 1)2 - (x - 2)2 + (y - 1)2 + 4

⇒ x2 + 1 - 2 + y2 + 1 + 2y + z2 - x2 + 4 - 4x + y2 + 1 - 2y + 4

⇒ 2x + 4y = 7 ... (i)

(BP)2 = (CP)2 ⇒ (x - 2)+ (y - 1)2 + 4 = (x - 3)2 + (y - 2)2 + 1

⇒ x+ 4 - 4x + y2 + 1 - 2y + z2 + 4 = x2 + 9 - 6x + y2 + 4 - 4y +1

⇒ 2x + 2y = 5 ... (ii)

(AP)2 = (CP)2 ⇒ (x - 1)2 + (y + 1)+ (x - 3)+ (y - 2)2 + 1

⇒ x+ 1 - 2x + y2 + 1 + 2y = x2 + 9 - 6x + y2 + 4 - 4y + 1

⇒ 4x + 5y = 12 ... (iii)

Solving equation (i) and (ii) we get

y = 1, $\text{x} = \frac{3}{2}$

Put x and y in equation (iii)

$4\Big(\frac{3}{2}\Big) + 6(1) = 12$

$12-12$

So, the required point is $\Big(\frac{3}{2},\ 1,\ 0\Big)$

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