Proportionality · TEKS 7.5A, 7.5C
Similar figures & scale drawings
Similar figures have the same shape. All side lengths are multiplied by the same scale factor.
Key steps
- Scale factor = new length ÷ matching original length.
- Missing side = known side × scale factor.
- Map and drawing scales: set up a proportion or find "miles per inch".
- Area scales by the scale factor SQUARED.
Worked example
A drawing room is 3 cm by 5 cm at 1 cm = 4 m. What is the real area?
- Real sides: 3 × 4 = 12 m and 5 × 4 = 20 m.
- Area = 12 × 20 = 240 m².
- Answer: 240 m² (not 15 × 4 = 60)
Watch out: Similar figures MULTIPLY, they don’t ADD. If one side grows from 6 to 9, the other side is ×1.5, not +3.
Practice
5 questions with new numbers every set. Get 4 of 5 to mark this topic as practiced.