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

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

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

I.

Question 1.
If $$|\overline{\mathbf{p}}|=2,|\overline{\mathbf{q}}|=3$$ and $$(\bar{p}, \bar{q})=\frac{\pi}{6}$$, then find $$|\bar{p} \times \bar{q}|^{2}$$.
Solution:

Question 2.
If $$\overline{\mathbf{a}}=2 \overline{\mathbf{i}}-\overline{\mathbf{j}}+\overline{\mathbf{k}}$$ and $$\overline{\mathbf{b}}=\overline{\mathbf{i}}-3 \overline{\mathbf{j}}-5 \overline{\mathbf{k}}$$, then find $$|\overline{\mathbf{a}} \times \overline{\mathbf{b}}|$$.
Solution:

Question 3.
If $$\bar{a}=2 \bar{i}-3 \bar{j}+\overline{\mathbf{k}}$$ and $$\bar{b}=\bar{i}+4 \bar{j}-2 \bar{k}$$, then find $$(\overline{\mathbf{a}}+\overline{\mathbf{b}}) \times(\overline{\mathbf{a}}-\overline{\mathbf{b}})$$.
Solution:

Question 4.
If $$4 \bar{i}+\frac{2 p}{3} \bar{j}+p \bar{k}$$ is parallel to the vector $$\overline{\mathbf{i}}+2 \overline{\mathbf{j}}+3 \overline{\mathbf{k}}$$, find p.
Solution:

Question 5.
Compute $$\overline{\mathbf{a}} \times(\overline{\mathbf{b}}+\overline{\mathbf{c}})+\overline{\mathbf{b}} \times(\overline{\mathbf{c}}+\overline{\mathbf{a}})+\overline{\mathbf{c}} \times(\overline{\mathbf{a}}+\overline{\mathbf{b}})$$
Solution:

Question 6.
If $$\overline{\mathbf{p}}=\mathbf{x} \overline{\mathbf{i}}+\mathbf{y} \overline{\mathbf{j}}+\mathbf{z} \overline{\mathbf{k}}$$, find the value of $$|\overline{\boldsymbol{p}} \times \overline{\mathbf{k}}|^{2}$$
Solution:

Question 7.
Compute $$2 \bar{j} \times(3 \bar{i}-4 \bar{k})+(\bar{i}+2 \hat{j}) \times \bar{k}$$.
Solution:

Question 8.
Find a unit vector perpendicular to both $$\overline{\mathbf{i}}+\overline{\mathbf{j}}+\overline{\mathbf{k}}$$ and $$2 \bar{i}+\bar{j}+3 \bar{k}$$.
Solution:

Question 9.
If θ is the angle between the vectors $$\overline{\mathbf{i}}+\overline{\mathbf{j}}$$ and $$\overline{\mathbf{j}}+\overline{\mathbf{k}}$$, then find sin ?.
Solution:

Question 10.
Find the area of the parallelogram having $$\bar{a}=2 \bar{j}-\bar{k}$$ and $$\overline{\mathbf{b}}=-\overline{\mathbf{i}}+\overline{\mathbf{k}}$$ as adjacent sides.
Solution:
Vector area of the parallelogram having $$\bar{a}=2 \bar{j}-\bar{k}$$ and $$\overline{\mathbf{b}}=-\overline{\mathbf{i}}+\overline{\mathbf{k}}$$ as adjacent sides.

Question 11.
Find the area of the parallelogram, whose diagonals are $$3 \overline{\mathbf{i}}+\overline{\mathbf{j}}-2 \overline{\mathbf{k}}$$ and $$\bar{i}-3 \bar{j}+4 \bar{k}$$.
Solution:

Question 12.
Find the area of the triangle having $$3 \bar{i}+4 \bar{j}$$ and $$-5 \bar{i}+7 \bar{j}$$ as two of its sides.
Solution:

Question 13.
Find unit vector perpendicular to the plane determined by the vectors $$\bar{a}=4 \bar{i}+3 \bar{j}-\bar{k}$$ and $$\overline{\mathbf{b}}=2 \tilde{i}-6 \overline{\mathbf{j}}-3 \overline{\mathbf{k}}$$.
Solution:

Question 14.
Find the area of the triangle whose vertices are A(1, 2, 3), B(2, 3, 1) and C(3, 1, 2).
Solution:
Suppose $$\bar{i}, \bar{j}, \bar{k}$$ are unit vectors along the co-ordinate axes.
Position vectors of A, B, C are

II.

Question 1.
If $$\overline{\mathbf{a}}+\overline{\mathbf{b}}+\overline{\mathbf{c}}=\overline{\mathbf{0}}$$, then prove that $$\overline{\mathbf{a}} \times \overline{\mathbf{b}}=\overline{\mathbf{b}} \times \overline{\mathbf{c}}=\overline{\mathbf{c}} \times \overline{\mathbf{a}}$$.
Solution:

Question 2.
If $$\overline{\mathbf{a}}=2 \bar{i}+\bar{j}-\bar{k}, \quad \bar{b}=-\bar{i}+2 \bar{j}-4 \bar{k}$$ and $$\overline{\mathbf{c}}=\overline{\mathbf{i}}+\mathbf{j}+\overline{\mathbf{k}}$$, then find $$(\bar{a} \times \bar{b}) \cdot(\bar{b} \times \bar{c})$$.
Solution:

Question 3.
Find the vector area and the area of the parallelogram having $$\overline{\mathbf{a}}=\overline{\mathbf{i}}+2 \overline{\mathbf{j}}-\overline{\mathbf{k}}$$ and $$\overline{\mathbf{b}}=2 \overline{\mathbf{i}}-\overline{\mathbf{j}}+\mathbf{2} \overline{\mathbf{k}}$$ as adjacent sides.
Solution:

Question 4.
If $$\overline{\mathbf{a}} \times \overline{\mathbf{b}}=\overline{\mathbf{b}} \times \overline{\mathbf{c}} \neq \overline{\mathbf{0}}$$, show that, $$\overline{\mathbf{a}}+\overline{\mathbf{c}}=\mathbf{p} \overline{\mathbf{b}}$$, where p is some scalar.
Solution:

Question 5.
Let $$\overline{\mathbf{a}}$$ and $$\overline{\mathbf{b}}$$ be vectors, satisfying $$|\overline{\mathbf{a}}|=|\overline{\mathbf{b}}|=5$$ and $$(\bar{a}, \bar{b})=45^{\circ}$$. Find the area of the triangle having $$\overline{\mathbf{a}}-\mathbf{2} \overline{\mathbf{b}}$$ and $$3 \bar{a}+2 \bar{b}$$ as two of its sides.
Solution:

Question 6.
Find the vector having magnitude ?6 units and perpendicular to both $$\mathbf{2} \overline{\mathbf{i}}-\overline{\mathbf{k}}$$ and $$\mathbf{3} \overline{\mathbf{i}}-\overline{\mathbf{j}}-\overline{\mathbf{k}}$$.
Solution:

Question 7.
Find a unit vector perpendicular to the plane determined by the points P(1, -1, 2), Q(2, 0, -1) and R(0, 2, 1).
Solution:

Question 8.
If $$\overline{\mathbf{a}} \cdot \overline{\mathbf{b}}=\overline{\mathbf{a}} \cdot \overline{\mathbf{c}}$$ and $$\overline{\mathbf{a}} \times \overline{\mathrm{b}}=\overline{\mathrm{a}} \times \overline{\mathrm{c}}, \overline{\mathbf{a}} \neq 0$$, then show that $$\overline{\mathbf{b}}=\overline{\mathbf{c}}$$.
Solution:

Question 9.
Find a vector of magnitude 3 and perpendicular to both the vector $$\overline{\mathbf{b}}=2 \bar{i}-2 \bar{j}+\bar{k}$$ and $$\bar{c}=2 \bar{i}+2 \bar{j}+3 \bar{k}$$.
Solution:

Question 10.
If $$|\overline{\mathbf{a}}|$$ = 13, $$|\overline{\mathbf{b}}|$$ = 5 and $$\overline{\mathbf{a}} \cdot \overline{\mathbf{b}}=\mathbf{6 0}$$, then find $$|\overline{\mathbf{a}} \times \overline{\mathbf{b}}|$$.
Solution:

Question 11.
Find a unit vector perpendicular to the plane passing through the points (1, 2, 3), (2, -1, 1) and (1, 2, -4).
Solution:
Let ‘O’ be the origin and let A, B, C be the given points.

III.

Question 1.
If $$\overline{\mathbf{a}}$$, $$\overline{\mathbf{b}}$$ and $$\overline{\mathbf{c}}$$ represent the vertices A, B and C respectively of ∆ABC, then prove that $$|(\overline{\mathbf{a}} \times \overline{\mathbf{b}})+(\overline{\mathbf{b}} \times \overline{\mathbf{c}})+(\overline{\mathbf{c}} \times \overline{\mathbf{a}})|$$ is twice the area of ∆ABC.
Solution:

Question 2.
If $$\overline{\mathbf{a}}=2 \overline{\mathbf{i}}+3 \overline{\mathbf{j}}+4 \overline{\mathbf{k}}, \overline{\mathbf{b}}=\overline{\mathbf{i}}+\overline{\mathbf{j}}-\overline{\mathbf{k}}$$ and $$\overline{\mathbf{c}}=\overline{\mathbf{i}}-\overline{\mathbf{j}}+\overline{\mathbf{k}}$$, then compute $$\overline{\mathbf{a}} \times(\overline{\mathbf{b}} \times \overline{\mathbf{c}})$$ and verify that it is perpendicular to $$\bar{a}$$.
Solution:

Question 3.
If $$\overline{\mathbf{a}}=7 \overline{\mathbf{i}}-2 \overline{\mathbf{j}}+3 \overline{\mathbf{k}}, \overline{\mathbf{b}}=2 \overline{\mathbf{i}}+8 \overline{\mathbf{k}}$$ and $$\overline{\mathbf{c}}=\overline{\mathbf{i}}+\overline{\mathbf{j}}+\overline{\mathbf{k}}$$ then compute $$\overline{\mathbf{a}} \times \mathbf{b}, \overline{\mathbf{a}} \times \overline{\mathbf{c}}$$ and $$\bar{a} \times(\bar{b}+\bar{c})$$. Verify whether cross product is distributive over vector addition.
Solution:

Question 4.
If $$\overline{\mathbf{a}}=\overline{\mathbf{i}}+\overline{\mathbf{j}}+\overline{\mathbf{k}}, \overline{\mathbf{c}}=\overline{\mathbf{j}}-\overline{\mathbf{k}}$$, then find vector $$\overline{\mathbf{b}}$$ such that $$\overline{\mathbf{a}} \times \overline{\mathbf{b}}=\overline{\mathbf{c}}$$ and $$\bar{a} \cdot \bar{b}=3$$.
Solution:

Question 5.
$$\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}$$ are three vectors of equal magnitudes and each of them is inclined at an angle of 60° to the others. If $$|\bar{a}+\bar{b}+\bar{c}|=\sqrt{6}$$, then find $$|\overline{\mathbf{a}}|$$.
Solution:

Question 6.
For any two vectors $$\bar{a}$$ and $$\bar{b}$$, show that $$\left(1+|\bar{a}|^{2}\right)\left(1+|\bar{b}|^{2}\right)$$ = $$|\mathbf{1}-\overline{\mathbf{a}} \cdot \overline{\mathbf{b}}|^{2}+|\overline{\mathbf{a}}+\overline{\mathbf{b}}+\overline{\mathbf{a}} \times \overline{\mathbf{b}}|^{2}$$
Solution:

Question 7.
If $$\overline{\mathbf{a}}, \overline{\mathbf{b}}, \overline{\mathbf{c}}$$ are unit vectors such that $$\overline{\mathbf{a}}$$ is perpendicular to the plane of $$\overline{\mathbf{b}}, \overline{\mathbf{c}}$$ and the angle between $$\overline{\mathbf{b}}$$ and $$\overline{\mathbf{c}}$$ is $$\frac{\pi}{3}$$, then find $$|\bar{a}+\bar{b}+\bar{c}|$$.
Solution:
Given that $$|\bar{a}|=|\bar{b}|=|\bar{c}|=1$$

Question 8.
$$\overline{\mathbf{a}}=3 \overline{\mathbf{i}}-\overline{\mathbf{j}}+2 \overline{\mathbf{k}}, \overline{\mathbf{b}}=-\overline{\mathbf{i}}+3 \overline{\mathbf{j}}+2 \overline{\mathbf{k}}$$, $$\bar{c}=4 \bar{i}+5 \bar{j}-2 \bar{k}$$ and $$\bar{d}=\bar{i}+3 \bar{j}+5 \bar{k}$$ then compute the following.
(i) $$(\overline{\mathbf{a}} \times \overline{\mathbf{b}}) \times(\bar{c} \times \bar{d})$$ and
(ii) $$(\overline{\mathbf{a}} \times \overline{\mathbf{b}}) \cdot \overline{\mathbf{c}}-(\overline{\mathbf{a}} \times \overline{\mathbf{d}}) \cdot \overline{\mathbf{b}}$$
Solution: