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Similar Triangles

AA, SAS, and SSS similarity theorems

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Similar Triangles

Definition

Two triangles are similar if:

  • All corresponding angles are congruent
  • All corresponding sides are proportional

Symbol: △ABC∼△DEF\triangle ABC \sim \triangle DEF

Similarity Postulates

AA (Angle-Angle)

If two angles of one triangle are congruent to two angles of another triangle, the triangles are similar.

Note: If two angles match, the third must also match (angle sum = 180°)

SAS (Side-Angle-Side)

If two sides of one triangle are proportional to two sides of another triangle AND the included angles are congruent, the triangles are similar.

SSS (Side-Side-Side)

If all three pairs of corresponding sides are proportional, the triangles are similar.

Scale Factor

The ratio of corresponding sides: k=side in triangle 1corresponding side in triangle 2k = \frac{\text{side in triangle 1}}{\text{corresponding side in triangle 2}}

Properties

If △ABC∼△DEF\triangle ABC \sim \triangle DEF with scale factor kk:

  • Perimeters are in ratio k:1k:1
  • Areas are in ratio k2:1k^2:1

Applications

  • Finding unknown lengths
  • Indirect measurement
  • Proving geometric relationships

📚 Practice Problems

1Problem 1easy

❓ Question:

Two triangles are similar. The sides of the first triangle are 3, 4, and 5 cm. The shortest side of the second triangle is 6 cm. Find the other two sides of the second triangle.

💡 Show Solution

Step 1: Identify corresponding sides: Shortest side of first triangle: 3 cm Shortest side of second triangle: 6 cm

Step 2: Find the scale factor: Scale factor = 6/3 = 2 The second triangle is 2 times larger

Step 3: Apply scale factor to other sides: Second side: 4 × 2 = 8 cm Third side: 5 × 2 = 10 cm

Step 4: Verify the similarity ratio: All ratios should be equal: 6/3 = 2 ✓ 8/4 = 2 ✓ 10/5 = 2 ✓

Answer: The other two sides are 8 cm and 10 cm

2Problem 2easy

❓ Question:

In △ABC\triangle ABC, ∠A=50°\angle A = 50° and ∠B=60°\angle B = 60°. In △DEF\triangle DEF, ∠D=50°\angle D = 50° and ∠E=60°\angle E = 60°. Are the triangles similar?

💡 Show Solution

Two angles of △ABC\triangle ABC are congruent to two angles of △DEF\triangle DEF:

  • ∠A≅∠D\angle A \cong \angle D (both 50°50°)
  • ∠B≅∠E\angle B \cong \angle E (both 60°60°)

By AA (Angle-Angle) similarity, the triangles are similar.

Answer: Yes, △ABC∼△DEF\triangle ABC \sim \triangle DEF by AA

3Problem 3easy

❓ Question:

Triangle ABC is similar to triangle DEF. If AB = 12, BC = 15, and DE = 8, find EF.

💡 Show Solution

Step 1: Identify corresponding sides: Since △ABC ~ △DEF: AB corresponds to DE BC corresponds to EF

Step 2: Find the scale factor: Scale factor = DE/AB = 8/12 = 2/3

Step 3: Set up proportion for unknown side: BC/EF = AB/DE 15/EF = 12/8

Step 4: Solve for EF: 15/EF = 3/2 3 × EF = 15 × 2 3 × EF = 30 EF = 10

Step 5: Verify with scale factor: EF = BC × (2/3) = 15 × (2/3) = 10 ✓

Answer: EF = 10

4Problem 4medium

❓ Question:

If △ABC∼△XYZ\triangle ABC \sim \triangle XYZ with sides AB=6AB = 6, BC=8BC = 8, AC=10AC = 10 and XY=9XY = 9, find YZYZ and XZXZ.

💡 Show Solution

Find the scale factor using corresponding sides: k=XYAB=96=32k = \frac{XY}{AB} = \frac{9}{6} = \frac{3}{2}

For YZ (corresponds to BC): YZBC=32\frac{YZ}{BC} = \frac{3}{2} YZ8=32\frac{YZ}{8} = \frac{3}{2} YZ=12YZ = 12

For XZ (corresponds to AC): XZAC=32\frac{XZ}{AC} = \frac{3}{2} XZ10=32\frac{XZ}{10} = \frac{3}{2} XZ=15XZ = 15

Answer: YZ=12YZ = 12, XZ=15XZ = 15

5Problem 5medium

❓ Question:

In similar triangles, the ratio of corresponding sides is 3:5. If the perimeter of the smaller triangle is 24 cm, what is the perimeter of the larger triangle?

💡 Show Solution

Step 1: Understand the property: In similar triangles, the ratio of perimeters equals the ratio of corresponding sides

Step 2: Set up the proportion: Perimeter ratio = Side ratio P_small/P_large = 3/5

Step 3: Substitute known value: 24/P_large = 3/5

Step 4: Cross multiply: 3 × P_large = 24 × 5 3 × P_large = 120 P_large = 40

Step 5: Verify: Ratio: 24/40 = 3/5 ✓

Answer: The perimeter of the larger triangle is 40 cm

6Problem 6medium

❓ Question:

Triangle ABC has sides 6, 8, and 10. Triangle DEF has sides 9, 12, and 15. Are these triangles similar? If so, what is the scale factor?

💡 Show Solution

Step 1: Order the sides from smallest to largest: Triangle ABC: 6, 8, 10 Triangle DEF: 9, 12, 15

Step 2: Check ratios of corresponding sides: Shortest sides: 9/6 = 3/2 = 1.5 Middle sides: 12/8 = 3/2 = 1.5 Longest sides: 15/10 = 3/2 = 1.5

Step 3: Analyze the ratios: All three ratios are equal to 3/2

Step 4: Conclusion: Since all corresponding side ratios are equal, the triangles ARE similar by SSS Similarity

Step 5: Identify scale factor: Scale factor = 3/2 or 1.5 Triangle DEF is 1.5 times larger than triangle ABC

Answer: Yes, the triangles are similar with scale factor 3/2 (or 1.5)

7Problem 7hard

❓ Question:

A tree casts a shadow 24 feet long at the same time a 6-foot person casts a 4-foot shadow. How tall is the tree?

💡 Show Solution

The sun creates similar triangles (same angle).

Set up proportion: tree heighttree shadow=person heightperson shadow\frac{\text{tree height}}{\text{tree shadow}} = \frac{\text{person height}}{\text{person shadow}}

h24=64\frac{h}{24} = \frac{6}{4}

Solve: 4h=1444h = 144 h=36h = 36

Answer: The tree is 36 feet tall

8Problem 8hard

❓ Question:

A tree casts a shadow 24 feet long at the same time a 6-foot person casts a shadow 4 feet long. How tall is the tree?

💡 Show Solution

Step 1: Identify the similar triangles: The tree and its shadow form a right triangle The person and their shadow form a right triangle Since the sun angle is the same, these triangles are similar

Step 2: Set up the proportion: Tree height/Tree shadow = Person height/Person shadow h/24 = 6/4

Step 3: Simplify the right side: h/24 = 3/2

Step 4: Cross multiply: 2h = 24 × 3 2h = 72 h = 36

Step 5: Alternative method - find scale factor: Scale factor = 24/4 = 6 Tree height = 6 × 6 = 36 feet

Step 6: Verify: 36/24 = 3/2 ✓ 6/4 = 3/2 ✓ The ratios are equal

Answer: The tree is 36 feet tall

Explain using:

📌 Related Topics in Triangles

❓ Frequently Asked Questions

What is Similar Triangles?▾
AA, SAS, and SSS similarity theorems
How can I study Similar Triangles effectively?▾
Start by reading the study notes and working through the examples on this page. Then use the flashcards to test your recall. Practice with the 8 problems provided, checking solutions as you go. Regular review and active practice are key to retention.
Is this Similar Triangles study guide free?▾
Yes — all study notes, flashcards, and practice problems for Similar Triangles on Study Mondo are free to access. No account is needed.
What course covers Similar Triangles?▾
Similar Triangles is part of the Geometry course on Study Mondo, specifically in the Triangles section. You can explore the full course for more related topics and practice resources.
Are there practice problems for Similar Triangles?▾
Yes, this page includes 8 practice problems with detailed solutions. Each problem includes a step-by-step explanation to help you understand the approach.