Special Right Triangles

45-45-90 and 30-60-90 triangles

Special Right Triangles

45-45-90 Triangle

An isosceles right triangle with angles 45ยฐ, 45ยฐ, and 90ยฐ.

Side Ratios: x:x:x2x : x : x\sqrt{2}

Where:

  • xx = length of each leg
  • x2x\sqrt{2} = length of hypotenuse

Key Pattern: If legs = 1, then hypotenuse = 2\sqrt{2}

Example: If each leg is 5, hypotenuse = 525\sqrt{2}

30-60-90 Triangle

A right triangle with angles 30ยฐ, 60ยฐ, and 90ยฐ.

Side Ratios: x:x3:2xx : x\sqrt{3} : 2x

Where:

  • xx = length of side opposite 30ยฐ (shortest side)
  • x3x\sqrt{3} = length of side opposite 60ยฐ
  • 2x2x = length of hypotenuse (opposite 90ยฐ)

Key Pattern: If short leg = 1, then long leg = 3\sqrt{3}, hypotenuse = 2

Example: If short leg is 6, long leg = 636\sqrt{3}, hypotenuse = 12

Why These Are Useful

  • Appear frequently in geometry problems
  • Can solve without Pythagorean Theorem
  • Used in trigonometry
  • Found in regular polygons and circles

Remember

45-45-90: legs equal, hypotenuse = leg ร— 2\sqrt{2}

30-60-90: hypotenuse = 2 ร— short leg, long leg = short leg ร— 3\sqrt{3}

๐Ÿ“š Practice Problems

1Problem 1easy

โ“ Question:

In a 45-45-90 triangle, one leg measures 6. Find the length of the other leg and the hypotenuse.

๐Ÿ’ก Show Solution

Step 1: Recall 45-45-90 triangle properties: This is an isosceles right triangle If each leg = x, then hypotenuse = xโˆš2

Step 2: Identify the given information: One leg = 6

Step 3: Find the other leg: In a 45-45-90 triangle, both legs are equal Other leg = 6

Step 4: Find the hypotenuse: Hypotenuse = leg ร— โˆš2 Hypotenuse = 6โˆš2

Step 5: Verify using Pythagorean Theorem: 6ยฒ + 6ยฒ = (6โˆš2)ยฒ 36 + 36 = 36 ร— 2 72 = 72 โœ“

Answer: Other leg = 6, Hypotenuse = 6โˆš2

2Problem 2easy

โ“ Question:

In a 45-45-90 triangle, each leg has length 8. Find the hypotenuse.

๐Ÿ’ก Show Solution

In a 45-45-90 triangle, the hypotenuse = leg ร— 2\sqrt{2}

Hypotenuse=82\text{Hypotenuse} = 8\sqrt{2}

Answer: 828\sqrt{2} (or approximately 11.31)

3Problem 3easy

โ“ Question:

In a 30-60-90 triangle, the shorter leg measures 5. Find the longer leg and the hypotenuse.

๐Ÿ’ก Show Solution

Step 1: Recall 30-60-90 triangle ratios: If shorter leg (opposite 30ยฐ) = x Then longer leg (opposite 60ยฐ) = xโˆš3 And hypotenuse = 2x

Step 2: Identify given information: Shorter leg = 5, so x = 5

Step 3: Find the longer leg: Longer leg = xโˆš3 = 5โˆš3

Step 4: Find the hypotenuse: Hypotenuse = 2x = 2(5) = 10

Step 5: Verify using Pythagorean Theorem: 5ยฒ + (5โˆš3)ยฒ = 10ยฒ 25 + 25(3) = 100 25 + 75 = 100 100 = 100 โœ“

Answer: Longer leg = 5โˆš3, Hypotenuse = 10

4Problem 4medium

โ“ Question:

In a 30-60-90 triangle, the hypotenuse is 20. Find both legs.

๐Ÿ’ก Show Solution

In a 30-60-90 triangle, sides are in ratio x:x3:2xx : x\sqrt{3} : 2x

Hypotenuse = 2x2x: 2x=202x = 20 x=10x = 10

Short leg (opposite 30ยฐ): x=10x = 10

Long leg (opposite 60ยฐ): x3=103x\sqrt{3} = 10\sqrt{3}

Answer: Short leg = 1010, Long leg = 10310\sqrt{3}

5Problem 5medium

โ“ Question:

In a 30-60-90 triangle, the hypotenuse is 20. Find both legs.

๐Ÿ’ก Show Solution

Step 1: Recall the ratio: Shorter leg : Longer leg : Hypotenuse = x : xโˆš3 : 2x

Step 2: Use the hypotenuse to find x: Hypotenuse = 2x 20 = 2x x = 10

Step 3: Find the shorter leg: Shorter leg = x = 10

Step 4: Find the longer leg: Longer leg = xโˆš3 = 10โˆš3

Step 5: Verify: 10ยฒ + (10โˆš3)ยฒ = 20ยฒ 100 + 100(3) = 400 100 + 300 = 400 400 = 400 โœ“

Answer: Shorter leg = 10, Longer leg = 10โˆš3

6Problem 6medium

โ“ Question:

In a 45-45-90 triangle, the hypotenuse is 8โˆš2. Find the length of each leg.

๐Ÿ’ก Show Solution

Step 1: Recall the relationship: In a 45-45-90 triangle: If leg = x, then hypotenuse = xโˆš2

Step 2: Set up the equation: xโˆš2 = 8โˆš2

Step 3: Solve for x: x = 8โˆš2 / โˆš2 x = 8

Step 4: Both legs equal x: Each leg = 8

Step 5: Verify: 8ยฒ + 8ยฒ = (8โˆš2)ยฒ 64 + 64 = 64 ร— 2 128 = 128 โœ“

Answer: Each leg = 8

7Problem 7hard

โ“ Question:

A 45-45-90 triangle has a hypotenuse of 12212\sqrt{2}. Find the area of the triangle.

๐Ÿ’ก Show Solution

Step 1: Find the leg length

In 45-45-90 triangle: hypotenuse = leg ร— 2\sqrt{2} legร—2=122\text{leg} \times \sqrt{2} = 12\sqrt{2} leg=12\text{leg} = 12

Step 2: Find area (both legs are equal) A=12ร—baseร—heightA = \frac{1}{2} \times \text{base} \times \text{height} A=12ร—12ร—12A = \frac{1}{2} \times 12 \times 12 A=72A = 72

Answer: Area = 7272 square units

8Problem 8hard

โ“ Question:

A square has a diagonal of length 10. Find the side length of the square and its perimeter.

๐Ÿ’ก Show Solution

Step 1: Recognize the special triangle: A square's diagonal divides it into two 45-45-90 triangles

Step 2: Set up the relationship: In a 45-45-90 triangle: If leg = s (side of square), then hypotenuse = sโˆš2 The diagonal is the hypotenuse

Step 3: Use the given diagonal: sโˆš2 = 10

Step 4: Solve for s: s = 10/โˆš2 s = 10/โˆš2 ร— โˆš2/โˆš2 (rationalize) s = 10โˆš2/2 s = 5โˆš2

Step 5: Find the perimeter: Perimeter = 4s = 4(5โˆš2) = 20โˆš2

Step 6: Verify the diagonal: Using Pythagorean theorem: sยฒ + sยฒ = diagonalยฒ (5โˆš2)ยฒ + (5โˆš2)ยฒ = 10ยฒ 25(2) + 25(2) = 100 50 + 50 = 100 โœ“

Answer: Side length = 5โˆš2, Perimeter = 20โˆš2