Mathematics
Grade9
Easy

Question

Find b in this 30-60-90 triangle.

  1. straight b equals 3.5 square root of 3
  2. b = 7.5
  3. b = 7
  4. b equals 7 square root of 3

hintHint:

We are given a right-angled triangle. We are given one of its side. It is a 30°-60°-90° triangle. We are asked to find the value of its other side. It is denoted by the variable “b”.

The correct answer is: b = 7


    Let the given triangle be ABC.
    ∠ABC = 90°
    ∠BAC = 60°
    BC = 7√3
    AB = b
    The sum of angles of a triangle is 180°.
    So, ∠BCA = 30°
    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. The side opposite to 60° is BC.
    Length of smallest side= b
    Length of longest side = 7√3
    So we can write,
    Length of longer side = √3(length of smaller side)
    BC = √3(AB)
    7√3 = √3(b)
    We will divide the both sides by √3 and rearranging the equation for b. b = 7√3 ÷ √3
    b = 7
    Therefore, the length of b is 7.

    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 using trigonometric ratios.

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