Circles and Geometric Shapes | Exercise 5.3

Question 3

Two parallel chords of lengths 6 cm6\text{ cm} and 8 cm8\text{ cm} are on opposite sides of the centre of a circle. If the radius of the circle is 5 cm5\text{ cm}, find the distance between the midpoints of the chords.

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Solution

We will find the distance of each chord from the center. Then we will add these distances.

Step 1 — Distance of first chord

Let the first chord be ABAB. Its length is 6 cm\mathbf{6 \text{ cm}}. The radius of the circle is 5 cm\mathbf{5 \text{ cm}}. The perpendicular from the center bisects the chord. So, half the chord length is 3 cm\mathbf{3 \text{ cm}}. Let d1d_1 be the distance from the center to chord ABAB. We use the Pythagorean theorem.

d12+32=52d_1^2 + 3^2 = 5^2

d12+9=25d_1^2 + 9 = 25

d12=259d_1^2 = 25 - 9

d12=16d_1^2 = 16

d1=4 cm\boxed{d_1 = 4 \text{ cm}}

Diagram 1

Step 2 — Distance of second chord

Let the second chord be CDCD. Its length is 8 cm\mathbf{8 \text{ cm}}. The radius of the circle is 5 cm\mathbf{5 \text{ cm}}. The perpendicular from the center bisects the chord. So, half the chord length is 4 cm\mathbf{4 \text{ cm}}. Let d2d_2 be the distance from the center to chord CDCD. We use the Pythagorean theorem.

d22+42=52d_2^2 + 4^2 = 5^2

d22+16=25d_2^2 + 16 = 25

d22=2516d_2^2 = 25 - 16

d22=9d_2^2 = 9

d2=3 cm\boxed{d_2 = 3 \text{ cm}}

Diagram 2

Step 3 — Total distance between midpoints

The chords are on opposite sides of the center. The distance between their midpoints is the sum of d1d_1 and d2d_2.

Distance=d1+d2\text{Distance} = d_1 + d_2

=4+3= 4 + 3

7 cm\boxed{7 \text{ cm}}

Answer

The distance between the midpoints of the two chords is 7 cm\mathbf{7 \text{ cm}}.

More questions in Exercise 5.3

Q1

Can you explain why the converse to Theorem 4 is true, i.e., why does the perpendicular from the centre of a circle to a chord of the circle bisect the chord?

(Hint: Use Fig. 5.12. You are told that CMA=CMB=90\angle CMA = \angle CMB = 90^\circ. You need to show that AM=BMAM = BM.)

Q2

An isosceles triangle ABC is inscribed in a circle, with AB=ACAB = AC. Show that the altitude from A to BC passes through the centre of the circle.

Q3

Two parallel chords of lengths 6 cm6\text{ cm} and 8 cm8\text{ cm} are on opposite sides of the centre of a circle. If the radius of the circle is 5 cm5\text{ cm}, find the distance between the midpoints of the chords.

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