# Inter 1st Year Maths 1A Products of Vectors Solutions Ex 5(a)

Practicing the Intermediate 1st Year Maths 1A Textbook Solutions Inter 1st Year Maths 1A Products of Vectors Solutions Exercise 5(a) will help students to clear their doubts quickly.

## Intermediate 1st Year Maths 1A Products of Vectors Solutions Exercise 5(a)

I.

Question 1.
Find the angle between the vectors $$\bar{i}+2 \bar{j}+3 \bar{k}$$ and $$3 \bar{i}-\bar{j}+2 \bar{k}$$.
Solution:
Let $$\overline{\mathrm{a}}=\overline{\mathrm{i}}+2 \overline{\mathrm{j}}+3 \overline{\mathrm{k}}$$ and $$\overline{\mathrm{b}}=3 \overline{\mathrm{i}}-\overline{\mathrm{j}}+2 \overline{\mathrm{k}}$$ and ‘θ’ be the angle between them (i.e.,) $$(\bar{a}, \bar{b})$$ = θ Question 2.
If the vectors $$\mathbf{2} \overline{\mathbf{i}}+\lambda \overline{\mathbf{j}}-\overline{\mathbf{k}}$$ and $$4 \bar{i}-2 \bar{j}+2 \bar{k}$$ are perpendicular to each other, then find λ.
Solution:  Question 3.
For what values of λ, the vectors $$\overline{\mathbf{i}}-\lambda \overline{\mathbf{j}}+2 \overline{\mathbf{k}}$$ and $$8 \overline{\mathbf{i}}+6 \overline{\mathbf{j}}-\overline{\mathbf{k}}$$ are at right angles?
Solution: Question 4.
$$\overline{\mathbf{a}}=2 \overline{\mathbf{i}}-\overline{\mathbf{j}}+\overline{\mathbf{k}}, \overline{\mathbf{b}}=\overline{\mathbf{i}}-3 \overline{\mathbf{j}}-5 \overline{\mathbf{k}}$$. Find the vector C such that $$\overline{\mathbf{a}}$$, $$\overline{\mathbf{b}}$$ and $$\overline{\mathbf{c}}$$ form the sides of a triangle.
Solution: Question 5.
Find the angle between the planes $$\bar{r} \cdot(2 \bar{i}-\bar{j}+2 \bar{k})=3$$ and $$\overline{\mathrm{r}} \cdot(3 \overline{\mathrm{i}}+6 \bar{j}+\bar{k})=4$$.
Solution: Question 6.
Let $$\overline{\mathbf{e}}_{1}$$ and $$\overline{\mathbf{e}}_{2}$$ be unit vectors makingangle θ. If $$\frac{1}{2}\left|\bar{e}_{1}-\bar{e}_{2}\right|=\sin \lambda \theta$$, then find λ.
Solution:   Question 7.
Let $$\overline{\mathbf{a}}=\overline{\mathbf{i}}+\overline{\mathbf{j}}+\overline{\mathbf{k}}$$ and $$\overline{\mathbf{b}}=\mathbf{2} \overline{\mathbf{i}}+3 \overline{\mathbf{j}}+\overline{\mathbf{k}}$$. Find
(i) The projection vector of $$\overline{\mathbf{b}}$$ on $$\overline{\mathbf{a}}$$ and its magnitude.
(ii) The vector components of $$\overline{\mathbf{b}}$$ in the direction of a and perpendicular to $$\overline{\mathbf{a}}$$.
Solution: Question 8.
Find the equation of the plane through the point (3, -2, 1) and perpendicular to the vector (4, 7, -4).
Solution: Question 9.
If $$\overline{\mathbf{a}}=2 \bar{i}+2 \bar{j}-3 \bar{k}$$; $$\overline{\mathbf{b}}=3 \overline{\mathbf{i}}-\overline{\mathbf{j}}+2 \overline{\mathbf{k}}$$, then find the angle between $$2 \overline{\mathbf{a}}+\overline{\mathbf{b}}$$ and $$\bar{a}+2 \bar{b}$$.
Solution: II.

Question 1.
Find unit vector parallel to the XOY- plane and perpendicular to the vector $$4 \bar{i}-3 \bar{j}+\bar{k}$$.
Solution:
Any vector parallel to XOY-plane will be of the form $$p \bar{i}+q \bar{j}$$
∴ The vector parallel to the XOY-plane and perpendicular to the vector $$4 \bar{i}-3 \bar{j}+\bar{k}$$ is $$3 \bar{i}+4 \bar{j}$$
Its magnitude = $$|3 \bar{i}+\overline{4 j}|=\sqrt{9+16}=5$$
∴ Unit vector parallel to the XOY-plane and perpendicular to the vector $$4 \bar{i}-3 \bar{j}+\bar{k}$$ is $$\pm \frac{(3 \overline{\mathrm{i}}+4 \overline{\mathrm{j}})}{5}$$ Question 2.
If $$\overline{\mathbf{a}}+\overline{\mathbf{b}}+\overline{\mathrm{c}}=0,|\overline{\mathbf{a}}|=3,|\overline{\mathbf{b}}|=5$$ and $$|\bar{c}|=7$$, then find the angle between $$\overline{\mathbf{a}}$$ and $$\overline{\mathbf{b}}$$.
Solution: Question 3.
If $$|\overline{\mathbf{a}}|$$ = 2, $$|\overline{\mathbf{b}}|$$ = 3 and $$|\overline{\mathbf{c}}|$$ = 4 and each of $$\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}$$ is perpendicular to the sum of the other two vectors, then find the magnitude of $$\overline{\mathbf{a}}+\overline{\mathbf{b}}+\overline{\mathbf{c}}$$.
Solution:  Question 4.
Find the equation of the plane passing through the point $$\overline{\mathbf{a}}=\mathbf{2} \overline{\mathbf{i}}+3 \bar{j}-\overline{\mathbf{k}}$$ and perpendicular to the vector $$3 \bar{i}-2 \bar{j}-2 \bar{k}$$ and the distance of this plane from the origin.
Solution:
Equation of the plane passing through the point $$\overline{\mathbf{a}}=\mathbf{2} \overline{\mathbf{i}}+3 \bar{j}-\overline{\mathbf{k}}$$ and perpendicular to the vector $$\bar{n}=3 \bar{i}-2 \bar{j}-2 \bar{k}$$ is  Question 5.
$$\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}$$ and $$\overline{\mathbf{d}}$$ are the position vectors of four coplanar points such that $$(\mathbf{a}-\overline{\mathbf{d}}) \cdot(\bar{b}-\bar{c})=(\bar{b}-\bar{d}) \cdot(\bar{c}-\bar{a})=0$$. Show that the point $$\bar{d}$$ represents the orthocentre of the triangle with $$\bar{a}$$, $$\bar{b}$$ and $$\bar{c}$$ as its vertices.
Solution:
Position vectors of A, B, C, D are $$\bar{a}$$, $$\bar{b}$$, $$\bar{c}$$ and $$\bar{d}$$ respectively.  ⇒ BD is perpendicular to AC
∴ BD is another altitude of ∆ABC.
Altitudes AD and BD intersect at D.
∴ D is the orthocentre of ∆ABC.

III.

Question 1.
Show that the points (5, -1, 1), (7, -4, 7), (1, -6, 10) and (-1, -3, 4) are the vertices of a rhombus.
Solution:
Let A(5, -1, 1), B(7, -4, 7), C(1, -6, 10) and D(-1, -3, 4) are the given points. ∵ AB = BC = CD = DA = 7 units
AC ≠ BD
∴ A, B, C, D points are the vertices of a rhombus. Question 2.
Let $$\bar{a}=4 \bar{i}+5 \bar{j}-\bar{k}, \quad \bar{b}=\bar{i}-4 \bar{j}+5 \bar{k}$$ and $$\overline{\mathbf{c}}=\mathbf{3} \overline{\mathbf{i}}+\overline{\mathbf{j}}-\overline{\mathbf{k}}$$. Find the vector which is perpendicular to both $$\overline{\mathbf{a}}$$ and $$\overline{\mathbf{b}}$$ and whose magnitude is twenty one times the magnitude of $$\overline{\mathbf{c}}$$.
Solution:  Question 3.
G is the centroid of ΔABC and a, b, c are the lengths of the sides BC, CA and AB respectively prove that a2 + b2 + c2 = 3 (OA2 + OB2 + OC2) – 9(OG)2 where O is any point.
Solution:
Given that $$\overline{\mathrm{BC}}=\overline{\mathrm{a}}, \overline{\mathrm{CA}}=\overline{\mathrm{b}}, \overline{\mathrm{AB}}=\overline{\mathrm{c}}$$
Let ‘O’ be the origin and let $$\overline{\mathrm{OA}}=\overline{\mathrm{p}}, \overline{\mathrm{OB}}=\overline{\mathrm{q}} \text { and } \overline{\mathrm{OC}}=\overline{\mathrm{r}}$$
Then P.V. of the centroid of ΔABC is From (1) and (2)
∴ a2 + b2 + c2 = 3(OA2 + OB2 + OC2) – 9(OG)2. Question 4.
A line makes angles θ1, θ2, θ3, and θ4 with the diagonals of a cube. Show that cos2θ1 + cos2θ2 + cos2θ3 + cos2θ4 = $$\frac{4}{3}$$.
Solution:   