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Assignment No 8 (3-D Geometry)

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Assignment No 8 (3-D Geometry)

1. For each of the following vectors, find its length and find a vector of length one (unit" vector)
parallel to it.
a = (4;-4; 2); b = (2; 1; 0) ; c = (4; 0; 1;-2; 0)

2. Find the distances between each of the following pairs of points


(a) (8;-4; 2), (-6; 1; 0); (b) (1; 1; 1), (5;-7;-7);
(c) (3; 0; 1; 4), (-2; 6; 1; 3);

3.A triangle has vertices A,B and C which have co-ordinate vectors (4; 1; 7); (7;-4; 6) and (6; 2; 8)
respectively. Find the lengths of the sides of the triangle and deduce that the triangle is right-angled.

4. A cube has vertices at the 8 points O (0; 0; 0), A(1; 0; 0), B (1; 1; 0), C (0; 1; 0), D(0; 0; 1),
E (1; 0; 1), F (1; 1; 1), G(0; 1; 1). Sketch the cube, and then find the angle between the diagonals
OF and AG .

4.Find the following projections:


(a) the projection of (2; 1; 4) on (1;-2; 1);
(b) the projection of (2;-1; 2) on (-1; 3; 0);
(c) the projection of (-2; 2; 7) on the direction of the line x = (1; 0; 2) + k(-1; 1; 2).

5. Find the areas of, and the normals to the planes of, the following parallelograms:
(a) the parallelogram spanned by (1; 3; 2) and (0; 2; 4);
(b) a parallelogram which has vertices at the three points A(0; 2; 1), B(-1; 3; 0) and C(3; 1; 2).

6. Find the areas of the triangles with the following vertices:


(a) A(0; 2; 1), B(-1; 3; 0) and C(3; 1; 2);
(b) A(2; 2; 0), B(-1; 0; 2) and C(0; 4; 3).

7. A tetrahedron has vertices A, B, C and D with coordinate vectors for the points being a =(0; 1; 2),
b = (-1; 4; 1), c = (1; 0; 3) and d = (-3; 1; 2). Find parametric vector equations for the two altitudes
of the tetrahedron which pass through the vertices A and B, and determine whether the two altitudes
intersect or not.
Note: An altitude of a tetrahedron through a vertex is a line through the vertex and perpendicular to
the opposite face.

8. Find the volumes of the following parallelepipeds:


(a) the parallelepiped spanned by (2; 1; 3), (4; 1; 2) and (0; 2; 1);
(b) a parallelepiped which has vertices at the four points A(2; 1; 3), B(-2; 1; 4), C(0; 4; 1) and
D(3;-1; 0).

9. Find parametric vector, point-normal, and Cartesian forms for the following planes:
(a) the plane through (1; 2;-2) perpendicular to (-1; 1; 2);
(b) the plane through (1; 2;-2) parallel to (-1; 1; 2) and (2; 3; 1);
(d) the plane through the three points (1; 2;-2), (-1; 1; 2) and (2; 3; 1);
(d) the plane with intercepts -1, 2 and -4 on the x , y and z axes;
10.
(a) Find the plane through (1; 2;-2) which is parallel to (-1; 2;-2)

(b) Find the line of intersection of the planes x .(1; 2; 3) = 0 and x = λ 1 (2,1,2) + λ 2 (1,0,1)

( c) Find the following projections:


(i) the projection of (2; 3; 8) on the normal to the plane 2x+2y+z 4
(ii) the projection of (2; 2;-1) on the line of intersection of the planes x ⋅ (1,−1,3) = 0 and
x = λ 1 (2,1,2) + λ 2 (3,1,−3)

11. Find the shortest distances between


(a) the point (-2; 1; 5) and the line x = (1; 2;-5) + k(6; 3;-4);
x −1 y−2 z −3
(b) the point (0; 3; 8) and the line = =
1 −1 4
(c) the point (11; 2; 1) and the line of intersection of the planes
x ⋅ (1,−1,3) = 0 and x = λ 1 (2,1,2) + λ 2 (3,1,−3)

12. Find the shortest distances between


(a) the point (2, 6,-5) and the plane ( x − (1, 2, 3)) ⋅ (−2, 4, 4) = 0
(b) the point (1, 4, 1) and the plane 2x-2y+y = 5
(c) the point (1, 2, 1) and the plane with intercepts at 3,-1,2 on the three axes;
(d) the origin and the plane through the three points (2, 1,3), (5,3,1) and (5,1,2).

13. Find the shortest distances between


(a) the line through (1,2, 3) parallel to (3 , 0, 1) and the line through the point (0, 2, 5) parallel to
(3,-2, 2);
(b) the line through the points (1, 3, 1) and (1, 5, -1) and the line through the points (0, 2, 1) and
(1, 2, -3),
x −1 y−2 z −3
(c) the lines x = (2, 7, 8) + k(4; 3;-5) and = =
− 10 1 4

14. Four points have coordinate vectors A(1; 0; 2), B(-1; 2; 3), C(0; 1; 1) and D(2; 0;-1).
(a) Find the parametric vector equations of the line through A and B and the line through C and D.
(b) Find the shortest distance between the lines AB and CD.
(c) Find the point P on AB and point Q on CD such that PQ is the shortest distance between the
lines AB and CD.

x−7 y+3 z−4


15. Show that the line = = intersects the planes 6x + 4y – 5z = 4 and
2 −1 1
x – 5y + 2z = 12 in the same point and deduce that the line is coplanar with the line of intersection
of the planes

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