Similarity · Grade 9
In Grade 9 you get fluent with: similar shapes and scale factors.
Similar shapes have the same shape but can be different sizes: their angles are equal and all their lengths are in the same ratio, the scale factor. Areas grow by the square of the scale factor and volumes by its cube.
Key facts
- Similar shapes: length scale factor k = new length ÷ old length.
- Equal angles (from parallel lines or vertically opposite angles) often show that triangles are similar.
Worked examples
Example 1
The triangles are similar. Find x.
Answer: x = 10 cm
Example 2
Two triangles are similar with a scale factor of 3. A side of the small triangle is 4 cm. How long is the matching side of the large one?
Answer: 12 cm
Example 3
Are triangles with sides 3, 4, 5 and 6, 8, 10 similar?
Answer: Yes (scale factor 2)
Example 4
A photo 10 cm wide and 15 cm tall is enlarged to 25 cm wide. How tall is it?
Answer: 37.5 cm