Finding the Cosine of the Angle Between Two Vectors

Oct 13, 2025 by wbrum.org 14 views

Introduction

In mathematics, vectors are used to represent quantities with both magnitude and direction. The cosine of the angle between two vectors is an important concept in vector calculus, as it can be used to determine the angle between two vectors, and it has numerous applications in physics, engineering, and other fields. In this article, we will discuss how to find the cosine of the angle between two vectors using the dot product formula.

The Dot Product Formula

The dot product formula is used to find the cosine of the angle between two vectors. It is given by:

\mathbf{u} \cdot \mathbf{v} = |\mathbf{u}| |\mathbf{v}| \cos \theta

where $\mathbf{u}$ and $\mathbf{v}$ are the two vectors, $|\mathbf{u}|$ and $|\mathbf{v}|$ are the magnitudes of the vectors, and $\theta$ is the angle between the vectors.

Finding the Cosine of the Angle Between Two Vectors

a. Finding the Cosine of the Angle Between Two Vectors

Let's consider two vectors $\mathbf{u} = 4\mathbf{i} + 3\mathbf{j}$ and $\mathbf{v} = 4\mathbf{i} - 3\mathbf{j}$ . We need to find the cosine of the angle between these two vectors.

First, we need to find the magnitudes of the vectors. The magnitude of a vector $\mathbf{u} = a\mathbf{i} + b\mathbf{j}$ is given by:

|\mathbf{u}| = \sqrt{a^2 + b^2}

So, the magnitudes of the vectors $\mathbf{u}$ and $\mathbf{v}$ are:

|\mathbf{u}| = \sqrt{4^2 + 3^2} = \sqrt{16 + 9} = \sqrt{25} = 5

|\mathbf{v}| = \sqrt{4^2 + (-3)^2} = \sqrt{16 + 9} = \sqrt{25} = 5

Now, we need to find the dot product of the vectors. The dot product of two vectors $\mathbf{u} = a\mathbf{i} + b\mathbf{j}$ and $\mathbf{v} = c\mathbf{i} + d\mathbf{j}$ is given by:

\mathbf{u} \cdot \mathbf{v} = ac + bd

So, the dot product of the vectors $\mathbf{u}$ and $\mathbf{v}$ is:

\mathbf{u} \cdot \mathbf{v} = (4)(4) + (3)(-3) = 16 - 9 = 7

Now, we can use the dot product formula to find the cosine of the angle between the vectors:

\cos \theta = \frac{\mathbf{u} \cdot \mathbf{v}}{|\mathbf{u}| |\mathbf{v}|} = \frac{7}{(5)(5)} = \frac{7}{25}

b. Finding the Cosine of the Angle Between Two Vectors

Let's consider two vectors $\mathbf{u} = -2\mathbf{i} + 3\mathbf{j}$ and $\mathbf{v} = 3\mathbf{i} + 2\mathbf{j}$ . We need to find the cosine of the angle between these two vectors.

First, we need to find the magnitudes of the vectors. The magnitudes of the vectors $\mathbf{u}$ and $\mathbf{v}$ are:

|\mathbf{u}| = \sqrt{(-2)^2 + 3^2} = \sqrt{4 + 9} = \sqrt{13}

|\mathbf{v}| = \sqrt{3^2 + 2^2} = \sqrt{9 + 4} = \sqrt{13}

Now, we need to find the dot product of the vectors. The dot product of the vectors $\mathbf{u}$ and $\mathbf{v}$ is:

\mathbf{u} \cdot \mathbf{v} = (-2)(3) + (3)(2) = -6 + 6 = 0

Now, we can use the dot product formula to find the cosine of the angle between the vectors:

\cos \theta = \frac{\mathbf{u} \cdot \mathbf{v}}{|\mathbf{u}| |\mathbf{v}|} = \frac{0}{(\sqrt{13})(\sqrt{13})} = 0

Conclusion

In this article, we discussed how to find the cosine of the angle between two vectors using the dot product formula. We considered two examples of vectors and found the cosine of the angle between them. The dot product formula is a powerful tool in vector calculus, and it has numerous applications in physics, engineering, and other fields.

References

[1] "Vector Calculus" by Michael Spivak
[2] "Calculus" by Michael Spivak
[3] "Vector Analysis" by William H. Young