Circles and Geometric Shapes | Exercise 5.4

Question 2

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

Question diagram 1
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Solution

We will show that chords equidistant from the center are equal in length.

Step 1 — Bisected chords

The line from the center is perpendicular to the chord. It bisects the chord into two equal parts. CE is perpendicular to AB. So, E is the midpoint of AB. This means AB=2AEAB = 2AE.

CH is perpendicular to GF. So, H is the midpoint of GF. This means GF=2GHGF = 2GH.

Diagram 1

Step 2 — Congruent triangles

Let's consider two triangles. These are CEA\triangle CEA and CHG\triangle CHG. CA and CG are radii of the same circle. So, CA=CGCA = CG. We are given that CE=CHCE = CH. CEA\angle CEA is a right angle. CHG\angle CHG is a right angle. So, CEA=CHG\angle CEA = \angle CHG. By RHS congruence rule, CEACHG\triangle CEA \cong \triangle CHG. Corresponding parts of congruent triangles are equal. Therefore, AE=GHAE = GH.

Step 3 — Equal chords

From Step 1, we know AB=2AEAB = 2AE. From Step 1, we know GF=2GHGF = 2GH. From Step 2, we found AE=GHAE = GH. Let's substitute GHGH for AEAE.

AB=2AEAB = 2AE

AB=2GHAB = 2GH

We also know GF=2GHGF = 2GH. Therefore,

AB=GF\boxed{AB = GF}

Answer

(i) AB = GF is shown.

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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