Students must practice these TS Inter 1st Year Maths 1A Important Questions Chapter 4 Addition of Vectors to help strengthen their preparations for exams.

## TS Inter 1st Year Maths 1A Addition of Vectors Important Questions

Question 1.

Show that the points whose position vectors are \(-2 \overline{\mathbf{a}}+3 \overline{\mathbf{b}}+5 \overline{\mathbf{c}}, \overline{\mathbf{a}}+2 \overline{\mathbf{b}}+3 \overline{\mathbf{c}}, 7 \overline{\mathbf{a}}-\overline{\mathbf{c}}\) are collinear when, \(\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}\) are non coplanar vectors.

Solution:

Let O be the origin of reference so that

Question 2.

If A B C D E F is a regular hexagon with centre o then prove that \(\overline{\mathrm{AB}}+\overline{\mathrm{AC}}+\overline{\mathrm{AD}}+\overline{\mathrm{AE}}+\overline{\mathrm{AF}}= \overline{3 \mathrm{AD}}=6 \overline{\mathrm{AO}}\).

Solution:

Question 3.

In the two dimensional plane, prove by using vector methods, the equation of the line whose intercepts on the axes are a and b is \(\frac{x}{a}+\frac{y}{b}\) = 1

Solution:

Let A = (a,0), B = (0,b). P = (x,y)

Let O be the origin so that

Question 4.

Find a unit vector in the direction of the vector = \(\overline{\mathbf{a}}=2 \overline{\mathbf{i}}+3 \overline{\mathbf{j}}+\overline{\mathrm{k}}\)

Solution:

The unit vector in the direction of the vector

Question 5.

Find a vector in the direction of vector \(\overline{\mathbf{a}}=\overline{\mathbf{i}}-2 \overline{\mathbf{j}}\) that has magnitude 7 units.

Solution:

The unit vector in the direction of given

Question 6.

Find the unit vector in the direction of sum of the vectors \(\overline{\mathbf{a}}=2 \overline{\mathbf{i}}+2 \overline{\mathbf{j}}-5 \overline{\mathbf{k}}\) and \( \overline{\mathbf{b}}=2 \overline{\mathbf{i}}+\overline{\mathbf{j}}+3 \overline{\mathbf{k}}\)

Solution:

The sum of the vectors is

Question 7.

Write the direction ratios of the vector \(\overline{\mathbf{a}}=\overline{\mathbf{i}}+\overline{\mathbf{j}}-\mathbf{2} \overline{\mathbf{k}}\) and hence calculate its direction cosines.

Solution:

Question 8.

Consider the two points P and Q with position vectors \(\overline{\mathrm{OP}}=3 \overline{\mathrm{a}}-2 \overline{\mathrm{b}}\) and \(\overline{\mathrm{OQ}}= \overline{\mathbf{a}}+\overline{\mathbf{b}}\). Find the position vector of a point R which divides the line joining P and Q in the ratio 2: 1 (i) internally and (ii) externally.

Solution:

i) The position vector of the point R dividing the joining of P and Q internally in the ratio 2: 1 is

\(\overline{\mathrm{OR}}=\frac{2(\overline{\mathrm{a}}+\overline{\mathrm{b}})+(3 \overline{\mathrm{a}}-2 \overline{\mathrm{b}})}{2+1}=\frac{5 \bar{a}}{3}\)

ii) The position vector of the point R dividing the joining of P and Q externally in the ratio 2: 1 is

\(\overline{\mathrm{OR}}=\frac{2(\overline{\mathrm{a}}+\overline{\mathrm{b}})-(3 \overline{\mathrm{a}}-2 \overline{\mathrm{b}})}{2-1}=4 \overline{\mathrm{b}}-\overline{\mathrm{a}}\)

Question 9.

Show that the points \(A(2 \bar{i}-\bar{j}+\bar{k}), \dot{B}(\overline{\mathbf{i}}-3 \overline{\mathbf{j}}-5 \overline{\mathbf{k}}), \mathbf{C}(3 \overline{\mathbf{i}}-4 \overline{\mathbf{j}}-4 \overline{\mathbf{k}})\) are the vertices of a right angled triangle.

Solution:

We have \(\overline{\mathrm{AB}}=\overline{\mathrm{OB}}-\overline{\mathrm{OA}}\)

∴ AB^{2} = BC^{2} + CA^{2} and hence a right angled triangle can be formed with the points A, B, and C.

Question 10.

Show that the points \(\mathbf{A}(2 \overline{\mathbf{i}}-\overline{\mathbf{j}}+\overline{\mathbf{k}}) \dot{B}(\bar{i}-3 \bar{j}-5 \bar{k}), C(3 \bar{i}-4 \bar{j}-4 \bar{k})\) are the vertices of a right angled triangle.

Solution:

Question 11.

In a ΔABC if \(\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}\) are the position vectors of the vertices A, B and C respectively, then prove that the position vector of the centroid G is \(\frac{1}{3}(\bar{a}+\bar{b}+\bar{c})\).

Solution:

Let G be the centroid of ΔABC and AD is the median through the vertex A.

Then AG : GD = 2: 1

Suppose \(\overline{\mathrm{OA}}=\overline{\mathrm{a}}, \overline{\mathrm{OB}}=\overline{\mathrm{b}}, \overline{\mathrm{OC}}=\overline{\mathrm{c}}\) with

reference to the specific origin O.

Mid point of BC is = \(\overline{\mathrm{OD}}=\frac{1}{2}(\overline{\mathrm{b}}+\overline{\mathrm{c}})\)

Since G divides AD in the ratio 2: 1 we have

Question 12.

In a ΔABC, If ‘O’ Is the circumcentre, and H is the orthocentre then show that

Solution:

Question 13.

Let \(\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}, \overline{\mathbf{d}}\) be the position vectors of A, B, C and D respectively which are the vertices of a tetrahedron. Then prove that the lines joining the vertices to the centroids of the opposite faces are concurrent. (this point is called the centroid or the centre of the tetrahedron)

Solution:

Let O be the origin and G_{1}, G_{2}, G_{3}, G_{4} be the centroids of ΔBCD, ΔCAD, ΔABD and ΔABC.

Then \(\overline{\mathrm{OG}}_1=\frac{\overline{\mathrm{b}}+\overline{\mathrm{c}}+\overline{\mathrm{d}}}{3}\)

Suppose P is the point which divides AG_{1} in the ratio 3: 1 then

Similarly position vectors of the points dividing BC_{2}, CG_{3} and DG_{4} in the ratio 3: 1 are each equal to \(\frac{\overline{\mathrm{a}}+\overline{\mathrm{b}}+\overline{\mathrm{c}}+\overline{\mathrm{d}}}{4}\)

Hence the point P lies on AG_{1}, BC_{2}, CG_{3}, DG_{4}.

Question 14.

Let OABC be a parallelogram and D is the midpoint of \(\overline{\mathbf{O A}}\). Prove that the segment is \(\overline{\mathbf{C D}}\) trisects the diagonal \(\overline{\mathbf{O B}}\) and is trisected by the diagonal\(\overline{\mathbf{O B}}\)

Solution:

Question 15.

Let \(\overline{\mathbf{a}}, \overline{\mathbf{b}}\) be non-collinear vectors, if

are such that 3α = 2β then find x and y.

Solution:

Question 16.

If the points whose position vectors are \(3 \overline{\mathbf{i}}-2 \overline{\mathbf{j}}-\overline{\mathbf{k}}, \quad 2 \overline{\mathbf{i}}+3 \overline{\mathbf{j}}-4 \overline{\mathbf{k}},-\overline{\mathbf{i}}+\overline{\mathbf{j}}+2 \mathbf{k} 4 \overline{\mathbf{i}}+5 \overline{\mathbf{j}}+\lambda \overline{\mathbf{k}}\) are coplanar, then show that \(\lambda=\frac{-146}{17}\)

Solution:

Let O be the origin and let A, B, C and D be the given points. Then

Question 17.

Find the equation of the line parallel to the vector \(2 \overline{\mathbf{i}}-\overline{\mathbf{j}}+2 \overline{\mathbf{k}}\) and which passes through the point A whose position vector is \(2 \bar{i}-\bar{j}+2 \bar{k}\). If P is a point on this line such that AP = 15, find the position vector of P.

Solution:

The vector equation of the line passing through the point \(\mathrm{A}(\bar{a})\) whose position vector is \(\overline{\mathrm{a}}=3 \overline{\mathrm{i}}+\overline{\mathrm{j}}-\overline{\mathrm{k}}\) and parallel to the vector \(\overline{\mathrm{b}}=2 \overline{\mathrm{i}}-\overline{\mathrm{j}}+2 \overline{\mathrm{k}}\) is \(\overline{\mathrm{r}}=\overline{\mathrm{a}}+\mathrm{t} \overline{\mathrm{b}}\) for some t ∈ R

Question 18.

Show that the line joining the pair of points \(6 \bar{a}-4 \bar{b}+4 \bar{c},-4 \bar{c}\) and the line joining the pair of points \(-\bar{a}-2 \bar{b}-3 \bar{c}, \bar{a}+2 \bar{b}-5 \bar{c}\) intersect at the point \(-4 \bar{c}\) when \(\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}\) are non coplanar vectors.

Solution:

The vector equation of the line joining points

and equation (5) is satisfied.

∴ The two lines intersect at the point from (1) is \(-4 \bar{c}\)

Question 19.

Find the point of intersection of the line \(\overline{\mathbf{r}}=\mathbf{2} \overline{\mathbf{a}}+\overline{\mathbf{b}}+\mathbf{t}(\overline{\mathbf{b}}-\overline{\mathbf{c}})\) and the plane \(\overline{\mathbf{r}}=\overline{\mathbf{a}}+\mathbf{x}(\overline{\mathbf{b}}+\overline{\mathbf{c}})+\mathbf{y}(\overline{\mathbf{a}}+2 \overline{\mathbf{b}}-\overline{\mathbf{c}})\) where a, b, c are non-coplanar vectors.

Solution:

Let \(\overline{\mathbf{r}}\) be the position vector of the point P the intersection of the line and the plane.

Question 20.

Prove that the vector equation of line through the points \(A(\bar{a}), B(\bar{b})\) is \(\overline{\mathbf{r}}=(\mathbf{1}-\mathbf{t}) \overline{\mathbf{a}}+\mathbf{t} \overline{\mathbf{b}}, \mathbf{t} \in \mathbf{R}\).

Solution:

Let O be the origin and P be any point on the line.