Question

Select all options that represents the side lengths of a 30 60 90 triangle.

Answer 1

Options (B) and (D)

Explanation:

If a triangle is a 30° - 60° - 90° triangle, measure of the angle between the leg and hypotenuse will be either 60° or 90°.

Therefore, by applying Cosine rule in the given options.

c² = a² + b² - 2abCosC

All the given options are the right triangles.

[Since they follow the Pythagoras theorem]

Option (A),

Angle between the sides having measures 3 and 5 units,

4² = 3² + 5² - 2(3)(5)CosC

16 = 9 + 25 - 30.CosC

30.CosC = 18

`C=\text{Cos}^(-1)((5)/(6) )`

C = 33.56°

Therefore, this triangle is not a 30-60-90 triangle.

Option (B),

Angle between the sides measuring 5 and 10 units,

`(5√(3))^2=5^2+10^2-2(5)(10)\text{CosC}`

75 = 125 - 100(CosC)

Cos(C) = 0.5

C = 60°

Therefore, other angles of the triangle will be 30° and 90°.

And it's a 30°-60°-90° triangle.

Option (C),

10, 10, 10√2

It's a right triangle and the measure of legs are equal.

Since legs of this sides are equal, angles opposite to these equal sides will be equal.

Sum of all interior angles = 180°

m∠A + m∠B + m∠C = 180°

If m∠B = 90°

m∠A + 90° + m∠C = 180°

2(m∠A) = 90°

m∠A = 45°

Therefore, the given sides make a 45°- 45°- 90° triangle.

Option (D).

Angle between the sides 3 and 6,

(3√3)² = 3² + 6² -2(3)(6)CosC

27 = 9 + 36 - 36.(CosC)

CosC =
`(18)/(36)`

C =
`\text{Cos}^(-1)((1)/(2))`

C = 60°

Therefore,

These sides make a 30°- 60°- 90° triangle.

Options (B) and (D) are the 30°- 60°- 90° triangle.