Projection of (3, 1) onto (1, 2) Step by Step

Step by step

The question

Question: find the projection of (3, 1) onto (1, 2).

v = [3; 1], s = [1; 2]

The projection of v onto s is the part of v that goes along s.

Think of the shadow of v on the line through s, with the light straight above.

projection: the part of v along s

like the shadow of v on the line of s

1 Write the rule

The rule: v dot s, over s dot s, times s.

The fraction is one number: how much of s we need.

proj = v • ss • s × s

2 Work out the number

The top: v • s, matching entries multiplied and added.

v • s = 3 × 1 + 1 × 2 = 3 + 2 = 5

The bottom: s • s.

s • s = 1 × 1 + 2 × 2 = 1 + 4 = 5

So the number is 5/5, which is 1.

number = 55 = 1

Careful!

The bottom is s dot s, the vector we project onto.

Not v dot v.

Careful: the bottom is s • s, not v • v

3 Times s

Times s: every entry times 1.

So the projection is (1, 2).

proj = 1 × [1; 2] = [1; 2]

4 Draw it

The line through s.

s goes to (1, 2).

v goes to (3, 1).

From the tip of v, straight down onto the line: at a right angle.

The projection ends where it lands: (1, 2).

5 Check

Let's check: the leftover, v minus the projection, must be at a right angle to s.

v − proj = [3; 1] − [1; 2]

v − proj = [3 − 1; 1 − 2]

v − proj = [2; −1]

Its dot product with s is 0.

So the leftover is at a right angle to s: the answer is right!

(v − proj) • s = 2 × 1 + (−1) × 2 = 2 − 2 = 0

Let's look back.

First: write the rule.

Then: work out the number.

Then: times s.

Then: draw it.

Last: check.

Answer