# Class 11 RD Sharma Solutions – Chapter 28 Introduction to 3D Coordinate Geometry – Exercise 28.2 | Set 2

**Question 13. Prove that the tetrahedron with vertices at points O(0,0,0), A(0,1,1), B(1,0,1) and C(1,1,0) is a regular one.**

**Solution:**

Given: The points O(0,0,0), A(0,1,1), B(1,0,1) and C(1,1,0).

A regular tetrahedron has all equal sides and diagonals.

We know that the distance between two points (a, b, c) and (d, e, f) is given as follows:

Clearly, OA = OB = OC = AB = BC = CA.

Hence O, A, B and C represent a regular tetrahedron.

**Question 14. Show that the points (3,2,2), (-1,1,3), (0,5,6), (2,1,2) lie on a sphere whose centre is (1,3,4). Also find its radius.**

**Solution:**

Given: The points A(3,2,2), B(-1,1,3), C(0,5,6), D(2,1,2) and E(1,3,4)

We know that the distance between two points (a, b, c) and (d, e, f) is given as follows:

= 3

= 3

= 3

= 3

Since EA = EB = EC = ED, the points lie on a sphere with centre E.

Radius of the sphere = 3 units.

**Question 15. Find the coordinates of the point which is equidistant from the four points O(0,0,0), A(2,0,0), B(0,3,0) and C(0,0,8).**

**Solution:**

Given: Points O(0,0,0), A(2,0,0), B(0,3,0) and C(0,0,8).

Let the required point be P(x,y,z).

We are given that OP = PA ⇒ OP

^{2}= PA^{2}Using formula, we have:

⇒ x

^{2}+ y^{2 }+ z^{2}= x^{2}− 4x + 4 + y^{2}+z^{2}⇒ 4x = 4

⇒ x = 1

Similarly, OP

^{2}= PB^{2}⇒

⇒ x

^{2}+ y^{2}+ z^{2}= x^{2}+ y^{2}− 6y +9 +z^{2}⇒ 6y = 9

⇒ y = 3/2

Also, OP

^{2}= PC^{2}⇒

⇒ x

^{2}+ y^{2}+ z^{2}= x^{2}+ y^{2}+z^{2}− 16z + 64⇒ 16z = 64

⇒ z = 4

Hence the point is P[1, 3/2, 4].

**Question 16. If A(-2,2,3) and B(13,-3,13) are two points, find the locus of a point P which moves in a way such that 3PA = 2PB.**

**Solution:**

Given: A(-2,2,3) and B(13,-3,13)

Let P = (x, y, z) be the required point.

We are given 3PA = 2PB

Using the formula,, we have:

Squaring both sides, we have;

9(x

^{2}+ 4x +4 + y^{2}+ 4 − 4y + z^{2}+ 9 − 6z) = 4(x^{2}+ 169 − 26x + y^{2}+9 + 6y + z^{2}+ 169 − 26z)

⇒ 5(x^{2}+ y^{2}+z^{2}) + 140x − 60y + 50z − 1235 = 0.

**Question 17. Find the locus of P if PA**^{2 }+ PB^{2} = 2k^{2}, where A and B are the points (3,4,5) and (-1,3,-7).

^{2 }+ PB

^{2}= 2k

^{2}, where A and B are the points (3,4,5) and (-1,3,-7).

**Solution:**

Given: A(3,4,5) and B(-1,3,-7)

Let P(x, y, z) be the required point.

PA

^{2}+ PB^{2}= 2k^{2}. Using the formula,we have:⇒ 2x

^{2}+ 2y^{2}+ 2z^{2}− 4x −14y + 4z + 109 − 2k^{2}= 0

⇒ 2(x^{2}+ y^{2}+z^{2}) − 4x −14y + 4z + 109 − 2k2 = 0.

**Question 18. Show that the points A(a, b, c), B(b, c, a) and C(c, a, b) are vertices of an equilateral triangle.**

**Solution:**

Given: points A(a, b, c), B(b, c, a) and C(c, a, b)

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

Since AB = BC = CA

ABC is an equilateral triangle.

**Question 19. Are points A(3,6,9), B(10,20,30)**,** and C(25,41,5) the vertices of a **right-angled** triangle?**

**Solution:**

Given: A(3,6,9), B(10,20,30) and C(25,41,5)

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

⇒ AB

^{2 }= 586⇒ BC

^{2 }= 4571⇒ CA

^{2 }= 2709Since, AB

^{2}+ BC^{2}≠ AC^{2}AB

^{2}+ AC^{2 }≠ BC^{2}BC

^{2}+ AC^{2}≠ AB^{2}

ABC is not a right triangle.

**Question 20. Verify that:**

**(i) (0,7,-10), (1,6,-6) and (4,9,-6) are the vertices of an isosceles triangle.**

**Solution:**

Given: A(0,7,-10), B(1,6,-6) and C(4,9,-6)

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

= 6

Since AB = BC, ABC is an isosceles triangle.

**(ii) (0,7,-10), (-1,6,6) and (4,9,-6) are the vertices of a right-angled triangle.**

**Solution:**

Given: Given: A(0,7,-10), B(1,6,-6) and C(4,9,-6)

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

= 6

Since AB^{2}+ BC^{2}= AC^{2}, ABC is a right triangle.

**(iii) (-1,2,1), (1,-2,5), (4,-7,8) and (2,-3,4) are the vertices of a parallelogram.**

**Solution:**

Given: A(-1,2,1), B(1,-2,5), C(4,-7,8) and D(2,-3,4)

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

Since the opposite sides are equal, ABCD is a parallelogram.

**(iv) (5,-1,1), (7,-4,7), (1,-6,10) and (-1,-3,4) are vertices of a rhombus.**

**Solution:**

Given: A(5,-1,1), B(7,-4,7), C(1,-6,10) and D(-1,-3,4)

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

Since AB = BC = CA = AD

ABCD is a rhombus.

**Question 21. Find the locus of the points which are equidistant from the points (1,2,3) and (3,2,-1).**

**Solution:**

Let P(x, y, z) be the point equidistant from the points A(1,2,3) and B(3,2,-1).

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

⇒ AP = BP or AP

^{2}= BP^{2}⇒ (x − 1)

^{2}+ (y − 2)^{2}+ (z − 3)^{2}= (x − 3)^{2}+ (y − 2)^{2}+ (z + 1)^{2}⇒ 4x − 8z = 14 − 14

⇒ x − 2z = 0.

**Question 22. Show that the points A(1,2,3), B(-1.-2,-1), C(2,3,2) and D(7,4,6) are the vertices of a parallelogram ABCD.**

**Solution:**

Using formula, distance between two points (a, b, c) and (d, e, f) is given as follows:

Since the opposite sides are equal, ABCD is a parallelogram.

**Question 23: Find the locus of the point, the sum of whose distances from the points A(4,0,0) and B(-4,0,0) is equal to 10.**

**Solution:**

Let P(x, y, z) be the required locus.

Given: PA + PB = 10. Using distance formula,

Squaring both sides, we get

16x

^{2}+ 625 + 200x = 25(x^{2}+ y^{2 }+ z^{2 }+ 8x + 16)

⇒ 9x^{2}+ 25y^{2}+ 25z^{2}– 225 = 0

**Question 24. Find the equation of the set of points P such that its distances from the points A(3,4,-5) and B(-2,1,4) are equal.**

**Solution:**

Given: A(3,4,-5) and B(-2,1,4)

Let P(x, y, z) be the required point, It is given that PA = PB.

Hence, PA

^{2}= PB^{2}Using distance formula, we have,

⇒ -6x + 9 – 8y + 16 + 10z + 25 = 4x + 4 – 2y +1 – 8z +16

⇒ 10x + 6y – 18z -29 = 0.