Mathematics
Grade9
Easy

Question

Find the length HK of of right triangle GHK.

  1. 300
  2. 0
  3. 150
  4. 17.3

hintHint:

We are given a right-angled triangle GHK. 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 HK. To solve this question, we will use the properties of a right-angled triangle.

The correct answer is: 17.3


    Let the point where altitude meets be “J”
    From the figure, we can write the values of lengths and angles.
    Length of GJ = 5
    Length of JK = 12
    Angle GHK = 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 GJH ~ HJK
    So, the ratio of their sides will be equal.   
    fraction numerator G J over denominator H J end fraction equals fraction numerator H J over denominator J K end fraction
5 over a equals a over 15
a to the power of 2 space end exponent equals space 5 space cross times space 15
a to the power of 2 space end exponent equals space 75
T a k i n g space t h e space s q u a r e space r o o t
a space equals space 8.66
    If we see, HK is a hypotenuse of the ∆HJK
    We will use Pythagoras to solve it further. Pythagoras theorem states that, the square of the hypotenuse is equal to sum of the square of the other sides.
    HK2 = HJ2+ JK2
    HK2 = 8.62 + 152
    = 79.99 + 225
    = 299.9
    Taking square roots
    HK = 17.3
    Therefore, the length of the HK is 17.3.

    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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