Circles and Geometric Shapes | Exercise 5.5

Question 1

Find the length of the chord of a circle where the radius is 7 cm and perpendicular distance is 6 cm.

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Solution

The perpendicular from the center of a circle to a chord bisects the chord.

Step 1 — Note the given values

Let's identify what we know. The radius of the circle is 7 cm. The perpendicular distance from the center to the chord is 6 cm.

Diagram 1

Step 2 — Find half the chord length

We can form a right-angled triangle. The radius is the hypotenuse. The perpendicular distance is one leg. Half the chord is the other leg. Let the half-chord be AM. We use the Pythagorean theorem.

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

72=62+AM27^2 = 6^2 + AM^2

49=36+AM249 = 36 + AM^2

AM2=4936AM^2 = 49 - 36

AM2=13AM^2 = 13

AM=13 cm\boxed{AM = \sqrt{13} \text{ cm}}

Step 3 — Calculate the full chord length

The perpendicular bisects the chord. So, the full chord length is twice AM. Let the chord be AB.

AB=2×AMAB = 2 \times AM

AB=2×13AB = 2 \times \sqrt{13}

AB=213 cm\boxed{AB = 2\sqrt{13} \text{ cm}}

Answer

The length of the chord is 213 cm2\sqrt{13} \text{ cm}.

More questions in Exercise 5.5

Q1

Find the length of the chord of a circle where the radius is 7 cm and perpendicular distance is 6 cm.

Q2

Explain why the following statement is true: If the perpendicular distance of a chord from the centre is dd and the radius is rr, then the chord length is 2r2d22\sqrt{r^2 - d^2}.

Q3

In a circle, if the distance of chord AB from the centre is twice the distance of another chord CD from the centre, then can we conclude that CD = 2 AB? Give reasons for your answer.

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