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Uses of the Scalar Product (CIE A Level Maths: Pure 3)
Revision Note
Uses of the Scalar Product
This revision note covers several applications of the scalar product for vectors – namely, how you can use the scalar product to:
- find the angle between vectors or lines
- test whether vectors or lines are perpendicular
- find the closest distance from a point to a line
How do I find the angle between two vectors?
- Recall that a formula for the scalar (or ‘dot’) between vectors and is
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- where is the angle between the vectors when they are placed ‘base to base’
- that is, when the vectors are positioned so that they start at the same point
- We arrange this formula to make the subject:
- To find the angle between two vectors
- Calculate the scalar product between them
- Calculate the magnitude of each vector
- Use the formula to find
- Use inverse trig to find
- where is the angle between the vectors when they are placed ‘base to base’
How do I find the angle between two lines?
- To find the angle between two lines, find the angle between their direction vectors
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- For example, if the lines have equations and , then the angle between the lines is given by
How do I tell if vectors or lines are perpendicular?
- Two (non-zero) vectors and are perpendicular if, and only if,
- If the a and b are perpendicular then:
- If then:
- a and b are perpendicular
- For example, the vectors and are perpendicular since
- If the a and b are perpendicular then:
How do I find the shortest distance from a point to a line?
- Suppose that we have a line with equation and a point not on
- Let be the point on which is closest to (sometimes called the foot of the perpendicular)
- Then the line between and will be perpendicular to the line
- To find the closest point
- Call and
- Since lies on , we have , for a unique real number
- Find the vector using
- is perpendicular to so form an equation using
- Solve this equation to find the value of
- Use the value of to find
- The shortest distance between the point and the line is the length
- Note that the shortest distance between the point and the line is sometimes referred to as the length of the perpendicular
Worked example
Examiner Tip
It can be easier and clearer to work with column vectors when dealing with scalar products.
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