Step by step
The question
Question: find the shadow x of the wall.
A 1.5 m boy has a 4.5 m shadow.
At the same time, the wall is 5 m tall.
1 Two similar triangles
The sun's rays come in at the same angle.
So the two dashed lines are parallel.
The boy and the wall 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 heights: 5 m and 1.5 m.
k is big over small.
Careful!
Never put a height with a shadow.
Height with height, shadow with shadow.
5 divided by 1.5 is 10/3.
The wall's triangle is 10/3 times bigger.
4 Find x
x is in the big triangle.
Its partner is the boy's shadow, 4.5 m.
So x is 4.5 times 10/3.
That is 15.
So the shadow of the wall is 15 m.
5 Check
Let's check: x with its partner.
15/4.5 is 10/3, the scale factor.
The boy is shorter than their shadow.
So the wall 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.