Step by step
The question
Question: find the height x of the building.
A 3 m pole has a 4 m shadow.
At the same time, the building has a 30 m shadow.
1 Two similar triangles
The sun's rays come in at the same angle.
So the two dashed lines are parallel.
The pole and the building both stand straight up: a right angle at the bottom.
Same angles, so the two triangles are similar.
One is a bigger copy of the other.
2 Match the sides
Matching sides go together.
The height goes with the height.
The shadow goes with the shadow.
3 Find the scale factor
The scale factor k says how many times bigger.
We know both shadows: 30 m and 4 m.
k is big over small.
Careful!
Never put a height with a shadow.
Shadow with shadow, height with height.
30 divided by 4 is 7.5.
The building's triangle is 7.5 times bigger.
4 Find x
x is in the big triangle.
Its partner is the pole's height, 3 m.
So x is 3 times 7.5.
That is 22.5.
So the height of the building is 22.5 m.
5 Check
Let's check: x with its partner.
22.5/3 is 7.5, the scale factor.
The pole is shorter than its shadow.
So the building is shorter than its shadow too.
It makes sense!
Let's look back.
First: two similar triangles.
Then: match the sides.
Then: find the scale factor.
Then: find x.
Last: check.