# Example 6 - Chapter 12 Class 11 Introduction to Three Dimensional Geometry

Last updated at May 29, 2018 by Teachoo

Last updated at May 29, 2018 by Teachoo

Transcript

Example 6 Find the equation of set of points P such that PA2 + PB2 = 2k2, where A and B are the points (3, 4, 5) and ( 1, 3, 7), respectively. Given A (3, 4, 5) & B ( 1, 3, 7) Let the Co-ordinate of point P be (x, y, z) We need to find equation of set of point P (x, y, z) Such that PA2 + PB2 = 2k2 First, we calculate (PA)2, (PB)2 Calculating (PA)2 P (x, y, z) , A (3, 4, 5) PA = x2 x1 2+ y2 y1 2+ z2 z1 2 Here, x1 = x, y1 = y, z1 = z x2 = 3, y2 = 4, z2 = 5 PA = 3 x 2+ 4 y 2+ 5 z 2 (PA)2 = 3 x 2+ 4 y 2+ 5 z 2 2 (PA)2 = (3 x)2 + (4 y)2 + (5 z)2 = (3)2 + (x)2 2(3)(x) + (4)2 + y2 2(4)(y) + (5)2 + (z)2 (5) (z) = 9 + x2 6x + 16 + y2 8y + 25 + z2 10z = x2 + y2 + z2 6x 8y 10z + 9 + 16 + 25 = x2 + y2 + z2 6x 8y 10z + 50 Calculating (PB)2 P (x, y, z) , B ( 1, 3, 7) PB = x2 x1 2+ y2 y1 2+ z2 z1 2 Here, x1 = x, y1 = y, z1 = z x2 = 1, y2 = 3, z2 = 7 PB = 1 x 2+ 3 y 2+ 7 z 2 Squaring both sides (PB)2 = 1 2 1+x 2 + 3 y 2+ 1)(7+z 2 (PB)2 = (1 + x)2 + (3 + y)2 + (7 + z)2 = (1)2 + (x)2 + 2(1)(x) + (3)2 + (y)2 2(3)(y) + (7)2 + (z)2 2(7) (z) = 1 + x2 + 2x + 9 + y2 6y + 49 + z2 + 14z Putting value of (PA)2 & (PB)2 in (1) (PA)2 + (PB)2 = 2k2 (x2 + y2 + z2 6x 8y + 4z + 50) + (x2 + y2 + z2 + 2x 6y + 14z + 59) = 2k2 2x2 + 2y2 + 2z2 4x 14y + 4z + 50 + 59 = 2k2 2x2 + 2y2 + 2z2 4x 14y + 4z = 2k2 50 59 2x2 + 2y2 + 2z2 4x 14y + 4z = 2k2 109 which is the required equation

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