Class 9 Mathematics

Chapter 5 Solutions — I'm Up and Down, and Round and Round

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Solutions

Overview

Step-by-step NCERT solutions for I'm Up and Down, and Round and Round (Chapter 5, NCERT Class 9 Mathematics) — the full working for every question, not just the final answer. You can also read the I'm Up and Down, and Round and Round textbook chapter.

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What these solutions cover

All 44 questions in I'm Up and Down, and Round and Round are solved in the PDF. Here's what's inside, exercise by exercise:

Circumcircle of a Triangle

Draw ΔABC with AB = 5 cm, ∠A = 70° and ∠B = 60°. Draw the circumcircle of ΔABC. Is the centre inside or outside the triangle?
Draw ΔABC with AB = 5 cm, ∠A = 100°, AC = 4 cm. Draw the circumcircle of ΔABC. Is the centre inside or outside the triangle?
Draw ΔABC, with AB = 6 cm, BC = 7 cm and CA = 7 cm. Draw the circumcircle of ΔABC. Let the circumcentre be O. Measure OA, OB, OC.
What is the least possible radius of a circle through two points A and B?

Chords and the Angles They Subtend

Show that the triangle formed by a chord and the centre of the circle is isosceles.
Show that if two such isosceles triangles (occurring in the previous question) have equal base length, they are congruent to each other.

Midpoints and Perpendicular Bisectors of Chords

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°. You need to show that AM = BM.)
An isosceles triangle ABC is inscribed in a circle, with AB = AC. Show that the altitude from A to BC passes through the centre of the circle.
Two parallel chords of lengths 6 cm and 8 cm are on opposite sides of the centre of a circle. If the radius of the circle is 5 cm, find the distance between the midpoints of the chords.

Distance of Chords from the Centre

Use the Baudhāyana–Pythagoras theorem to show why Theorem 6 must be true. (Theorem 6: Chords of a circle having the same length are all at the same distance from the centre.)
Consider Fig. 5.15. If CE is perpendicular to AB, CH is perpendicular to GF, and CE = CH, show that AB = GF. (Congruence method)
Solve the previous question using the Baudhāyana–Pythagoras theorem.

Distance of Chords from the Centre — Further Results

Find the length of the chord of a circle where the radius is 7 cm and perpendicular distance is 6 cm.
Explain why the following statement is true: If the perpendicular distance of a chord from the centre is d and the radius is r, then the chord length is 2√(r² − d²).
*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.

Angles Subtended by an Arc

In a circle with centre O, the central angle AOB is 60°. If the radius of the circle is 12 cm, what is the length of the chord AB?
2.
Let A and B be two points on a circle with centre O.
- (i) Are there points X, Y on the circle, on the same side of AB, such that ∠AXB is different from ∠AYB?
- (ii) Is it true that if ∠AXB = ∠AYB, then X and Y lie on the same side of the circle?
- (iii) If ∠AXB = ∠AYB, and X and Y do not lie on the circle, does the circle through A, B and X also pass through Y?
Find x in Fig. 5.26. (Figure shows points A, B, C, D on a circle; angle at D is 100°; angle at B is x.)

Exercises

In a circle, a chord is 5 cm away from the centre. If the radius of the circle is 13 cm, what is the length of the chord?
An arc of a circle subtends an angle of 70° at the centre. What is the measure of the angle subtended by the arc at a point on the circle?
The diameter of a circle is 26 cm. A chord of length 24 cm is drawn in the circle. Find the distance from the centre of the circle to the chord.
A circle has a radius of 15 cm. A chord is drawn. The distance from the centre of the circle to the chord is 9 cm. What is the length of the chord?
Prove that the perpendicular bisector of a chord passes through the centre of the circle.
The diameter of a circle is AB. Point C is on the circumference. What is the measure of the ∠ACB? Explain your reasoning.
ABCD is a cyclic quadrilateral inscribed in a circle. If ∠A measures 75°, what is the measure of ∠C? If ∠B measures 110°, what is the measure of ∠D?
Quadrilateral PQRS is inscribed in a circle. If ∠P = (2x + 10)° and ∠R = (3x − 20)°, find the value of x and the measures of ∠P and ∠R.
The distance of a chord of length 16 cm from the centre of a circle is 6 cm. Find the radius of the circle.
A cyclic quadrilateral has sides 5, 5, 12, 12 units. Find its area.
Consider a cyclic quadrilateral. Without drawing its circumcircle, how can we find out whether the centre of the circumcircle lies inside the quadrilateral or outside? What is the best way of finding out?
When two chords intersect, each of them is divided into two line segments. Show that if the intersecting chords are of equal length, then the line segments of one chord are equal to the corresponding line segments of the other chord.
Draw a circle in which a chord of 6 cm length stands at a distance of 3 cm from the centre. (Hint: Is it a circumcircle of a suitable triangle?)
Show that a rectangle is the only parallelogram that can be inscribed in a circle.
Show that if a rectangle is inscribed in a circle, then the point of intersection of its diagonals must lie at the centre of the circle.
Consider all chords of a circle of a fixed length. What is the shape formed by the midpoints of all these chords?
In a circle with centre O, chords AB and AC are congruent. Explain why this statement is true: "The centre of the circle lies on the angle bisector of ∠BAC".
Two parallel chords of lengths 10 cm and 24 cm are on the same side of the centre of a circle. The distance between the chords is 7 cm. Find the radius of the circle.
A regular hexagon is inscribed in a circle of radius r. Find the length of the sides of the hexagon and the distance of each side from the centre of the circle.
A quadrilateral MNOP is inscribed in a circle. If MN is a diameter, what can you say about ∠MOP and ∠MNP? Explain your reasoning.
Let ABCD be a cyclic quadrilateral. Explain why the exterior angle at any vertex is equal to the interior opposite angle (e.g., ∠CDE = ∠ABC, where E is a point on the extension of side CD).
"There is no chord of a circle that is longer than its diameter." How do you justify this statement?
Let A be any point within a given circle with centre O. Show that the shortest chord of the circle that passes through point A is the one that is perpendicular to OA.
How would you use the given figure (Fig. 5.30) to justify the statement that the angle in a semicircle is 90°? (In Fig. 5.30, A is the apex on the circle at the top, O is the centre on the diameter, the two diameter endpoints are unlabelled, a and b are the base angles at those two ends, and OA is drawn as a radius.)
In a circle, two chords CC' and DD' are drawn perpendicular to a diameter AB. Prove that the segment MM' joining the midpoints of the chords CD and C'D' is perpendicular to AB.
How would you use the given figure (Fig. 5.31) to justify the statement that the sum of the opposite angles of a cyclic quadrilateral is 180°? (In the figure, ABCD is cyclic with centre O; p, q are the parts of the angle at A/B and u, v are the parts of the angle at C/D formed by joining the vertices to O.)

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