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Question

ABC is a triangle right angled at B. D is a point on AC such that BD is perpendicular to AC. If AB = \(p\) and BC = \(\sqrt{3}p\), then what is BD equal to?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
\(\sqrt{3}p/2\)

Understanding the Right Triangle Problem

We are given a triangle named ABC, which has a right angle at vertex B. A point D is located on the hypotenuse AC such that the line segment BD is perpendicular to AC. This means BD is the altitude from the right angle to the hypotenuse. We are provided with the lengths of the two legs of the triangle:

  • AB = \(p\)
  • BC = \(\sqrt{3}p\)

Our goal is to find the length of the altitude BD.

Calculating the Hypotenuse AC

Since triangle ABC is a right-angled triangle at B, we can use the Pythagorean theorem to find the length of the hypotenuse AC. The theorem states that the square of the hypotenuse (AC) is equal to the sum of the squares of the other two sides (AB and BC).

Applying the theorem: \(AC^2 = AB^2 + BC^2\) Substitute the given values: \(AC^2 = (p)^2 + (\sqrt{3}p)^2\) \(AC^2 = p^2 + 3p^2\) \(AC^2 = 4p^2\) Now, take the square root of both sides to find AC: \(AC = \sqrt{4p^2}\) \(AC = 2p\)

Finding the Altitude BD using Triangle Area

There are two ways to calculate the area of triangle ABC:

  1. Using the two legs (AB and BC) as base and height.
  2. Using the hypotenuse (AC) as the base and the altitude (BD) as the height.

Let's calculate the area using the first method: Area \(= \frac{1}{2} \times \text{base} \times \text{height}\) Area \(= \frac{1}{2} \times AB \times BC\) Area \(= \frac{1}{2} \times p \times (\sqrt{3}p)\) Area \(= \frac{\sqrt{3}p^2}{2}\)

Now, let's use the second method, equating it to the area calculated above: Area \(= \frac{1}{2} \times AC \times BD\) \(\frac{\sqrt{3}p^2}{2} = \frac{1}{2} \times (2p) \times BD\)

To find BD, we can rearrange the equation: \(\frac{\sqrt{3}p^2}{2} = p \times BD\) \(BD = \frac{\sqrt{3}p^2}{2p}\) Simplify the expression by canceling out one \(p\): \(BD = \frac{\sqrt{3}p}{2}\)

Alternative Method: Similar Triangles

When an altitude is drawn from the right angle to the hypotenuse in a right-angled triangle, it divides the triangle into two smaller triangles that are similar to the original triangle and to each other. So, we have \(\triangle ABC \sim \triangle ADB \sim \triangle BDC\).

Let's use the similarity between \(\triangle ABC\) and \(\triangle ADB\): The ratio of corresponding sides are equal. \(\frac{AB}{AC} = \frac{BD}{BC}\) Substitute the known values: \(\frac{p}{2p} = \frac{BD}{\sqrt{3}p}\) \(\frac{1}{2} = \frac{BD}{\sqrt{3}p}\) Now, solve for BD: \(BD = \frac{1}{2} \times \sqrt{3}p\) \(BD = \frac{\sqrt{3}p}{2}\)

Both methods yield the same result for the length of the altitude BD.

Conclusion

The length of the altitude BD is \(\frac{\sqrt{3}p}{2}\).

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