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Ever tried to push a door and seen how it opens easily when you push in the right direction? That's dot product in real life. In Class 12 Maths Chapter 10 Exercise 10.3, you will find out how the angle between the vectors determines the outcome a useful concept in physics, maths, and even basic motion.
Class 12 Maths chapter 10 Exercise 10.3 solutions of NCERT are simplified and easy to understand, with step-by-step explanations for every question. Practising these will help you grasp the dot product more clearly and give you confidence during exams. You can find all NCERT solution for Class 12 chapter 10 exercises combined from the link below for easy revision.
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Get simple and clear NCERT solutions for Class 12 Maths Chapter 10 Exercise 10.3 to understand dot product of vectors in an easy way.
Answer:
Given
$\left | \vec a \right |=\sqrt{3}$
$\left | \vec b \right |=2$
$\vec a . \vec b = \sqrt 6$
As we know
$\vec a . \vec b = \left | \vec a \right |\left | \vec b \right |cos\theta$
where $\theta$ is the angle between two vectors
So,
$cos\theta =\frac{\vec a.\vec b}{\left | \vec a \right |\left | \vec b \right |}=\frac{\sqrt{6}}{\sqrt{3}*2}=\frac{1}{\sqrt{2}}$
$\theta=\frac{\pi}{4}$
Hence the angle between the vectors is $\frac{\pi}{4}$ .
Answer:
Given two vectors
$\vec a=\hat i - 2 \hat j + 3 \hat k \: \:and \: \: \vec b=3 \hat i - 2 \hat j + \hat k$
Now As we know,
The angle between two vectors $\vec a$ and $\vec b$ is given by
$\theta=cos^{-1}\left ( \frac{\vec a.\vec b}{\left | \vec a \right |\left | \vec b \right |}\right )$
Hence the angle between $\vec a=\hat i - 2 \hat j + 3 \hat k \: \:and \: \: \vec b=3 \hat i - 2 \hat j + \hat k$
$\theta=cos^{-1}\left ( \frac{(\hat i-2\hat j+3\hat k).(3\hat i-2\hat j+\hat k)}{\left | \hat i-2\hat j+3\hat k \right |\left |3\hat i-2\hat j+\hat k \right |}\right )$
$\theta=cos^{-1}\left ( \frac{3+4+3}{\sqrt{1^2+(-2)^2+3^3}\sqrt{3^2+(-2)^2+1^2}} \right )$
$\theta=cos^{-1}\frac{10}{14}$
$\theta=cos^{-1}\frac{5}{7}$
Question 3: Find the projection of the vector $\hat i - \hat j$ on the vector $\hat i + \hat j$
Answer:
Let
$\vec a=\hat i - \hat j$
$\vec b=\hat i + \hat j$
Projection of vector $\vec a$ on $\vec b$
$\frac{\vec a.\vec b}{\left | \vec b \right |}=\frac{(\hat i-\hat j)(\hat i+\hat j)}{\left |\hat i+\hat j \right |}=\frac{1-1}{\sqrt{2}}=0$
Hence, Projection of vector $\vec a$ on $\vec b$ is 0.
Question 4: Find the projection of the vector $\hat i + 3 \hat j + 7 \hat k$ on the vector $7\hat i - \hat j + 8 \hat k$
Answer:
Let
$\vec a =\hat i + 3 \hat j + 7 \hat k$
$\vec b=7\hat i - \hat j + 8 \hat k$
The projection of $\vec a$ on $\vec b$ is
$\frac{\vec a.\vec b}{\left | \vec b \right |}=\frac{(\hat i+3\hat j+7\hat k)(7\hat i-\hat j+8\hat k)}{\left | 7\hat i-\hat j+8\hat k \right |}=\frac{7-3+56}{\sqrt{7^2+(-1)^2+8^2}}=\frac{60}{\sqrt{114}}$
Hence, projection of vector $\vec a$ on $\vec b$ is
$\frac{60}{\sqrt{114}}$
Answer:
Given
$\\\vec a=\frac{1}{7}( 2 \hat i + 3 \hat j + 6 \hat k ), \\\ \vec b =\frac{1}{7}( 3 \hat i- 6 \hat j + 2 \hat k ),\\\vec c = \frac{1}{7}( 6\hat i + 2 \hat j -3\hat k )$
Now magnitude of $\vec a,\vec b \:and\: \vec c$
$\left | \vec a \right |=\frac{1}{7} \sqrt{2^2+3^2+6^2}=\frac{\sqrt{49}}{7}=1$
$\left | \vec b \right |=\frac{1}{7} \sqrt{3^2+(-6)^2+2^2}=\frac{\sqrt{49}}{7}=1$
$\left | \vec c \right |=\frac{1}{7} \sqrt{6^2+2^2+(-3)^2}=\frac{\sqrt{49}}{7}=1$
Hence, they all are unit vectors.
Now,
$\vec a.\vec b=\frac{1}{7}(2\hat i+3\hat j+6\hat k)\frac{1}{7}(3\hat i-6\hat j+2\hat k)=\frac{1}{49}(6-18+12)=0$
$\vec b.\vec c=\frac{1}{7}(3\hat i-6\hat j+2\hat k)\frac{1}{7}(6\hat i+2\hat j-3\hat k)=\frac{1}{49}(18-12-6)=0$
$\vec c.\vec a=\frac{1}{7}(6\hat i+2\hat j-3\hat k)\frac{1}{7}(2\hat i+3\hat j-6\hat k)=\frac{1}{49}(12+6-18)=0$
Hence all three are mutually perpendicular to each other.
Answer:
Given in the question
$( \vec a + \vec b ). ( \vec a - \vec b )=8$
$\left | \vec a \right |^2-\left | \vec b \right |^2=8$
Since $|\vec a |\: \:= 8 \: \:|\vec b |$
$\left | \vec {8b} \right |^2-\left | \vec b \right |^2=8$
$\left | \vec {63b} \right |^2=8$
$\left | \vec {b} \right |^2=\frac{8}{63}$
$\left | \vec {b} \right |=\sqrt{\frac{8}{63}}$
So, answer of the question is
$\left | \vec {a} \right |=8\left | \vec {b} \right |=8\sqrt{\frac{8}{63}}$
Question 7: Evaluate the product $( 3\vec a - 5 \vec b ). ( 2 \vec a + 7 \vec b )$ .
Answer:
To evaluate the product $( 3\vec a - 5 \vec b ). ( 2 \vec a + 7 \vec b )$
$( 3\vec a - 5 \vec b ). ( 2 \vec a + 7 \vec b )=6\vec a.\vec a+21\vec a.\vec b-10\vec b.\vec a-35\vec b.\vec b$
$=6\vec a.^2+11\vec a.\vec b-35\vec b^2$
$=6\left | \vec a \right |^2+11\vec a.\vec b-35\left | \vec b \right |^2$
Answer:
Given two vectors $\vec a \: \: and \: \: \vec b$
$\left | \vec a \right |=\left | \vec b\right |$
$\vec a.\vec b=\frac{1}{2}$
Now Angle between $\vec a \: \: and \: \: \vec b$
$\theta=60^0$
Now As we know that
$\vec a.\vec b=\left | \vec a \right |\left | \vec b \right |cos\theta$
$\frac{1}{2}=\left | \vec a \right |\left | \vec a \right |cos60^0$
$\left | a \right |^2=1$
Hence, the magnitude of two vectors $\vec a \: \: and \: \: \vec b$
$\left | a \right |=\left | b \right |=1$
Question 9: Find $|\vec x |$ , if for a unit vector $\vec a , ( \vec x -\vec a ) . ( \vec x + \vec a ) = 12$
Answer:
Given in the question that
$( \vec x -\vec a ) . ( \vec x + \vec a ) = 12$
And we need to find $\left | \vec x \right |$
$\left | \vec x \right |^2-\left | \vec a \right |^2 = 12$
$\left | \vec x \right |^2-1 = 12$
$\left | \vec x \right |^2 = 13$
$\left | \vec x \right | = \sqrt{13}$
So the value of $\left | \vec x \right |$ is $\sqrt{13}$
Answer:
Given in the question is
$\vec a = 2 \hat i + 2 \hat j + 3 \hat k , \vec b = - \hat i + 2 \hat j + \hat k \: \: and \: \: \vec c = 3 \hat i + \hat j$
and $\vec a + \lambda \vec b$ is perpendicular to $\vec c$
and we need to find the value of $\lambda$ ,
so the value of $\vec a + \lambda \vec b$ -
$\vec a + \lambda \vec b=2\hat i +2\hat j +3\hat k+\lambda (-\hat i+2\hat j+\hat k)$
$\vec a + \lambda \vec b=(2-\lambda)\hat i +(2+2\lambda)\hat j +(3+\lambda)\hat k$
As $\vec a + \lambda \vec b$ is perpendicular to $\vec c$
$(\vec a + \lambda \vec b).\vec c=0$
$((2-\lambda)\hat i +(2+2\lambda)\hat j +(3+\lambda)\hat k)(3\hat i+\hat j)=0$
$3(2-\lambda)+2+2\lambda=0$
$6-3\lambda+2+2\lambda=0$
$\lambda=8$
the value of $\lambda=8$ ,
Answer:
Given in the question that -
$\vec a \: \: \: and \: \: \vec b$ are two non-zero vectors
According to the question
$\left ( |\vec a | \vec b + |\vec b | \vec a\right )\left (|\vec a | \vec b - |\vec b | \vec a \right )$
$=|\vec a |^2 |\vec b|^2 - |\vec b |^2 |\vec a|^2+|\vec b||\vec a|\vec a.\vec b-|\vec a||\vec b|\vec b.\vec a=0$
Hence $|\vec a | \vec b + |\vec b | \vec a$ is perpendicular to $|\vec a | \vec b - |\vec b | \vec a$ .
Question 12: If $\vec a . \vec a = 0 \: \: and \: \: \vec a . \vec b = 0$ , then what can be concluded about the vector $\vec b$ ?
Answer:
Given in the question
$\\\vec a . \vec a = 0 \\|\vec a|^2=0$
$\\|\vec a|=0$
Therefore $\vec a$ is a zero vector. Hence any vector $\vec b$ will satisfy $\vec a . \vec b = 0$
Answer:
Given in the question
$\vec a , \vec b , \vec c$ are unit vectors $\Rightarrow |\vec a|=|\vec b|=|\vec c|=1$
and $\vec a + \vec b + \vec c = \vec 0$
and we need to find the value of $\vec a . \vec b + \vec b. \vec c + \vec c . \vec a$
$(\vec a + \vec b + \vec c)^2 = \vec 0$
$\vec a^2 + \vec b^2 + \vec c ^2+2(\vec a . \vec b + \vec b. \vec c + \vec c . \vec a)=0$
$|\vec a|^2 + |\vec b|^2 + |\vec c |^2+2(\vec a . \vec b + \vec b. \vec c + \vec c . \vec a)=0$
$1+1+1+2(\vec a . \vec b + \vec b. \vec c + \vec c . \vec a)=0$
$\vec a . \vec b + \vec b. \vec c + \vec c . \vec a=\frac{-3}{2}$
Answer- the value of $\vec a . \vec b + \vec b. \vec c + \vec c . \vec a$ is $\frac{-3}{2}$
Answer:
Let
$\vec a=\hat i-2\hat j +3\hat k$
$\vec b=5\hat i+4\hat j +1\hat k$
we see that
$\vec a.\vec b=(\hat i-2\hat j +3\hat k)(5\hat i+4\hat j +1\hat k)=5-8+3=0$
we now observe that
$|\vec a|=\sqrt{1^2+(-2)^2+3^2}=\sqrt{14}$
$|\vec b|=\sqrt{5^2+4^2+1^2}=\sqrt{42}$
Hence here converse of the given statement is not true.
Answer:
Given points,
A=(1, 2, 3),
B=(–1, 0, 0),
C=(0, 1, 2),
As need to find Angle between $\overline{BA}\: \: and\: \: \overline{BC} ]$
$\vec {BA}=(1-(-1))\hat i+(2-0)\hat j+(3-0)\hat k=2\hat i+2\hat j+3\hat k$
$\vec {BC}=(0-(-1))\hat i+(1-0)\hat j+(2-0)\hat k=\hat i+\hat j+2\hat k$
Hence angle between them ;
$\theta=cos^{-1}(\frac{\vec {BA}.\vec {BC}}{\left | \vec {BA} \right |\left | \vec {BC} \right |})$
$\theta=cos^{-1}\frac{2+2+6}{\sqrt{17}\sqrt{6}}$
$\theta=cos^{-1}\frac{10}{\sqrt{102}}$
Answer - Angle between the vectors $\overline{BA}\: \: and\: \: \overline{BC}$ is $\theta=cos^{-1}\frac{10}{\sqrt{102}}$
Question 16: Show that the points A(1, 2, 7), B(2, 6, 3) and C(3, 10, –1) are collinear.
Answer:
Given in the question
A=(1, 2, 7), B=(2, 6, 3) and C(3, 10, –1)
To show that the points A(1, 2, 7), B(2, 6, 3) and C(3, 10, –1) are collinear
$\vec {AB}=(2-1)\hat i+(6-2)\hat j+(3-7)\hat k$
$\vec {AB}=\hat i+4\hat j-4\hat k$
$\vec {BC}=(3-2)\hat i+(10-6)\hat j+(-1-3)\hat k$
$\vec {BC}=\hat i+4\hat j-4\hat k$
$\vec {AC}=(3-1)\hat i+(10-2)\hat j+(-1-7)\hat k$
$\vec {AC}=2\hat i+8\hat j-8\hat k$
$|\vec {AB}|=\sqrt{1^2+4^2+(-4)^2}=\sqrt{33}$
$|\vec {BC}|=\sqrt{1^2+4^2+(-4)^2}=\sqrt{33}$
$|\vec {AC}|=\sqrt{2^2+8^2+(-8)^2}=2\sqrt{33}$
As we see that
$|\vec {AC}|=|\vec {AB}|+|\vec {BC}|$
Hence point A, B , and C are colinear.
Answer:
Given the position vector of A, B , and C are
$2 \hat i - \hat j + \hat k , \hat i - 3 \hat j - 5 \hat k \: \: and \: \: 3 \hat i - 4 \hat j - 4 \hat k$
To show that the vectors $2 \hat i - \hat j + \hat k , \hat i - 3 \hat j - 5 \hat k \: \: and \: \: 3 \hat i - 4 \hat j - 4 \hat k$ form the vertices of a right angled triangle
$\vec {AB}=(1-2)\hat i + (-3-(-1))\hat j+(-5-1)\hat k=-1\hat i -2\hat j-6\hat k$
$\vec {BC}=(3-1)\hat i + (-4-(-3))\hat j+(-4-(-5))\hat k=-2\hat i -\hat j+\hat k$
$\vec {AC}=(3-2)\hat i + (-4-(-1))\hat j+(-4-(1))\hat k=\hat i -3\hat j-5\hat k$
$|\vec {AB}|=\sqrt{(-1)^2+(-2)^2+(-6)^2}=\sqrt{41}$
$|\vec {BC}|=\sqrt{(-2)^2+(-1)^2+(1)^2}=\sqrt{6}$
$|\vec {AC}|=\sqrt{(1)^2+(-3)^2+(-5)^2}=\sqrt{35}$
Here we see that
$|\vec {AC}|^2+|\vec {BC}|^2=|\vec {AB}|^2$
Hence A,B, and C are the vertices of a right angle triangle.
$\\A ) \lambda = 1 \\\\ B ) \lambda = -1 \\\\ C ) a = |\lambda | \\\\ D ) a = 1 / |\lambda |$
Answer:
Given $\vec a$ is a nonzero vector of magnitude ‘a’ and $\lambda$ a nonzero scalar
$\lambda \vec a$ is a unit vector when
$|\lambda \vec a|=1$
$|\lambda|| \vec a|=1$
$| \vec a|=\frac{1}{|\lambda|}$
Hence the correct option is D.
Product of Two Vectors
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Frequently Asked Questions (FAQs)
The main three topics are the addition of vectors, the dot product of vectors and the cross product of vectors.
The work done is the dot product of force and displacement. The dot product of two vectors is a scalar (real number).
The dot product of force and velocity gives power.
i.k=0 as the angle between them is 90 degrees
Yes, the dot product of two vectors can be either positive, negative or zero based on the angle between them.
There are a total of 5 exercises including miscellaneous.
Yes. For two perpendicular vectors, the dot product is zero.
On Question asked by student community
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Yes, if you miss the 1st CBSE exam due to valid reasons, then you can appear for the 2nd CBSE compartment exam.
From the academic year 2026, the board will conduct the CBSE 10th exam twice a year, while the CBSE 12th exam will be held once, as per usual. For class 10th, the second phase exam will act as the supplementary exam. Check out information on w hen the CBSE first exam 2026 will be conducted and changes in 2026 CBSE Board exam by clicking on the link .
If you want to change your stream to humanities after getting a compartment in one subject in the CBSE 12th Board Exam , you actually have limited options to qualify for your board exams. You can prepare effectively and appear in the compartment examination for mathematics again. If you do not wish to continue with the current stream, you can take readmission in the Humanities stream and start from Class 11th again, and continue studying for two more years to qualify for the 12th examination.
The GUJCET Merit List is prepared based on the Class 12th marks and GUJCET marks received by the students. CBSE students who are not from the Gujarat board can definitely compete with GSEB students, as their eligibility is decided based on the combined marks scored by them in GUJCET and the 12th board. The weightage of the GUJCET score is 40% and the weightage of the class 12 scores is 60%.
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