Mathematics
Grade9
Easy

Question

The long leg of a 30 - 60 - 90 is 18√3 units. Find the length of the hypotenuse.

  1. 36 units
  2. 18 units
  3. 18square root of 3 units
  4. 18square root of 2 units

hintHint:

We are given a right-angled triangle. It is 30°-60°-90° triangle. We are given the length of its long leg. It is 18√3 units. We are asked to find it’s hypotenuse.

The correct answer is: 36 units


    Let the given triangle be ABC.
    ∠ABC = 90°
    ∠CAB = 60°
    ∠BCA = 30°
    BC = 18√3
    It is 30°-60°-90° triangle.
    In a 30°-60°-90° triangle, the length of hypotenuse is two times the length of the smallest side. It’s longer side is √3 times the value of the smallest side.
    The side opposite to the 30° angle is the smallest side.
    The side opposite to the 60° angle is the longer side.
    In the given question, the side opposite to 30° is AB. And the side opposite to 60° is BC.
    Length of long leg = 18√3 units
    So we can write,
    Length of longer leg = √3(length of smaller leg)
    BC = √3(AB)
    So, to find the smaller leg we have to divide the longer side by √3.
    AB = BC ÷ √3
    AB = 18√3 ÷ √3
    AB = 18 units.
    Now,
    Hypotenuse = 2(length of small leg)
    AC = 2(18)
    AC = 36 units.
    Therefore, the length of the hypotenuse is 36 units.

    For such questions, we should know about the properties of a right-angled triangle and 30°-60°-90° triangle. The alternate way to solve the above question is by Pythagoras theorem and trigonometric ratios.

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