# Precalculus : Find the Measure of an Angle Between Two Vectors

## Example Questions

### Example Question #1 : Parallel And Perpendicular Vectors In Two Dimensions

Find the angle between the following two vectors in 3D space.

Explanation:

We can relate the dot product, length of two vectors, and angle between them by the following formula:

So the dot product of

and

is the addition of each product of components:

now the length of the vectors of a and b can be found using the formula for vector magnitude:

So:

hence

### Example Question #1 : Find The Measure Of An Angle Between Two Vectors

The dot product may be used to determine the angle between two vectors.

Use the dot product to determine if the angle between the two vectors.

,

Explanation:

First, we note that the dot product of two vectors is defined to be;

.

First, we find the left side of the dot product:

.

Then we compute the lengths of the vectors:

.

We can then solve the dot product formula for theta to get:

Substituting the values for the dot product and the lengths will give the correct answer.

### Example Question #1 : Find The Measure Of An Angle Between Two Vectors

Find the angle between the two vectors:

Explanation:

Solving the dot product formula for the angle between the two vectors results in the equation .

If we call the vectors a and b, finding the dot product and the lengths of the vectors, then substituting them into the formula will give the correct angle.

Substituting the values correctly will give the correct answer.

### Example Question #3 : Find The Measure Of An Angle Between Two Vectors

Find the measure of the angle between the following vectors:

Explanation:

To find the angle between two vectors, use the following formula:

is known as the dot product of two vectors. It is found via the following formula:

The denominator of the fraction involves multiplying the magnitude of each vector. To find the magnitude of a vector, use the following formula:

Now we have everything we need to find our answer. Use our given vectors:

So the angle between these two vectors is 102.09 degrees

### Example Question #4 : Find The Measure Of An Angle Between Two Vectors

Find the measure of the angle between the two vectors.

Explanation:

We use the dot product to find the angle between two vectors. The dot product has two formulas:

We solve for the angle measure to find the computational formula:

Our vectors give dot products and lengths:

Substituting these values into the formula above will give the correct answer.

### Example Question #6 : Parallel And Perpendicular Vectors In Two Dimensions

Find the angle between the vectors and .

Explanation:

To determine the angle between our two vectors, we can use the fact that for any 2 vectors and , where is the magnitude and is the angle between the 2 vectors, which is what we are looking for.

Working from the left, we can first find the dot product,

Now we'll find the magnitudes of the two vectors by using the Pythagorean Theorem:

take the square root of both sides

Now we can plug these values back into the equation to start solving for theta:

multiply the two numbers inside the radicals:

divide both sides by

take the inverse cosine of both sides

### Example Question #2 : Matrices And Vectors

Find the angle between the vectors  and . Note that the first vector is in polar form and the second is in component form.

Explanation:

To find the angle between two vectors, we can use the fact that . In order to find the dot product, we need to convert the vector to component form. This is easiest to do after drawing a quick sketch of the vector:

To find the vertical component, set up an equation involving sine, since the vertical component is the side of a right triangle across from the 20-degree angle:

evaluate the sine of 20, then multiply by 13

To find the horizontal component, set up an equation involving cosine, since the horizontal component is the side of a right triangle adjacent to the 20-degree angle:

evaluate the cosine of 20, then multiply by 13

The component form of this vector can be written as

Next, we can find the magnitude of the vector already in component form by using the Pythagorean Theorem:

take the square root of both sides

Now we have all the information that we need to solve for theta.

Find the dot product for the left side:

which we can now put back in the equation:

now divide both sides by

take invese cosine of both sides

### Example Question #2 : Find The Measure Of An Angle Between Two Vectors

Find  where  and . Note that is in component form while is in polar form.