Mathematics
Grade9
Easy

Question

  • If AD=2 and CD=5,  BD=?

  1. 17
  2. 3.16
  3. 2.4
  4. 25.12

hintHint:

We are given a right-angled triangle ABC. An altitude is drawn from the vertex having the right angle. It divides the base of the triangle into two parts. The values of parts are given. We are asked to find the value of the BD. To solve this question, we will use the properties of a right-angled triangle.

The correct answer is: 3.16


    The point where altitude meets be “D”
    From the figure, we can write the values of lengths and angles.
    Length of AD = 2
    Length of CD = 5
    Angle ABC = 90°
    Let the value of altitude be “a”.
    Due to the altitude, two right-angled triangles are formed.
    There is a theorem for altitude drawn from the right angle of a right-angled triangle. It states that, “When altitude is drawn from a right angle, two similar triangles are formed. They are similar to each other. They are also similar to the parent triangle”.
    Triangle ADB ~ BDC
    So, the ratio of their sides will be equal. 
    fraction numerator A D over denominator B D end fraction equals fraction numerator B D over denominator D C end fraction
2 over a space equals a over 5
a squared space equals space 5 space cross times space 2
a to the power of 2 space end exponent equals 10
a space equals space 3.16
    Therefore, the length of the BD is 3.16.

    To solve such questions, we should know the properties of right-angled triangles and similar triangles. To find the altitude, we can just remember that the square of the altitude is equal to product of the two values

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