Circles and Geometric Shapes | Exercise 5.4

Question 1

Use the Baudhāyana–Pythagoras theorem to show why Theorem 6 must be true.

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Solution

Let's use the Baudhāyana–Pythagoras theorem to prove that equal chords are equidistant from the circle's center.

Step 1 — Set up the diagram and knowns

Let's consider a circle with center O. We draw two chords, AB and CD. We are given that AB and CD have the same length. Let's draw a perpendicular from O to AB, meeting at M. Let's draw a perpendicular from O to CD, meeting at N. We know that a perpendicular from the center bisects the chord. So, M is the midpoint of AB, and N is the midpoint of CD.

AM=AB2AM = \frac{AB}{2}

CN=CD2CN = \frac{CD}{2}

We know that AB = CD. Therefore, their halves must also be equal.

AM=CN\boxed{AM = CN}

Also, OA and OC are radii of the same circle.

OA=OC\boxed{OA = OC}

Diagram 1

Step 2 — Apply the Baudhāyana–Pythagoras theorem

Now, let's look at the right-angled triangle OMA. The Baudhāyana–Pythagoras theorem states that hypotenuse2=side12+side22hypotenuse^2 = side1^2 + side2^2. Here, OA is the hypotenuse.

OA2=OM2+AM2OA^2 = OM^2 + AM^2

Next, let's look at the right-angled triangle ONC. Here, OC is the hypotenuse.

OC2=ON2+CN2OC^2 = ON^2 + CN^2

From Step 1, we know that OA = OC. So, their squares must also be equal.

OA2=OC2OA^2 = OC^2

Now, we can substitute the expressions from the Baudhāyana–Pythagoras theorem.

OM2+AM2=ON2+CN2OM^2 + AM^2 = ON^2 + CN^2

From Step 1, we also know that AM = CN. So, we can replace CN2CN^2 with AM2AM^2.

OM2+AM2=ON2+AM2OM^2 + AM^2 = ON^2 + AM^2

Let's subtract AM2AM^2 from both sides of the equation.

OM2=ON2OM^2 = ON^2

Taking the square root of both sides, we get:

OM=ON\boxed{OM = ON}

Answer

Since OM and ON represent the distances of the chords AB and CD from the center O, and we have shown that OM = ON, it proves that chords of a circle having the same length are at the same distance from the center.

More questions in Exercise 5.4

Q1

Use the Baudhāyana–Pythagoras theorem to show why Theorem 6 must be true.

Q2

Consider Fig. 5.15. If CE is perpendicular to AB, CH is perpendicular to GH, and CE = CH, show that AB = GF.

Q3

Solve the previous question using the Baudhāyana–Pythagoras theorem.

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