Tag: Circles – Exercise 4.4.2 – Class IX

Circles – Exercise 4.4.2 – Class IX

  1. In a circle whose radius is 8cm., chord is drawn at a point 3cm. from the centre of the circle. The chord is divided into two segments by a point on it. If one segment of the chord is 9cm .What is the length of the other segment?

Solution:

Circles – Exercise 4.4.2 – Class IX

From the data given,

Radius, OB = 8 cm

OC = 3cm

AB is a chord, The chord is divided into two line segments AP and BP , let BP = 9 cm. Now we have to find AP,

To find AP:

Proof:

In Δ OCB ; ∟OCB = 90˚[since OC⊥AB]

BC2 + OC2 = OB2

BQ2 = OB2 – OC2

BQ = √(82 – 32) = √(64-9) = √55

AB = 2√55

AP = AB – BP = (2√55 – 9)cm


  1. Suppose two chords of a circle are equidistant from the center of the circle. Prove that the chords have equal length.

Solution:

Circles – Exercise 4.4.2 – Class IX

Figure is drawn by the data given in the problem. Let O be the centre of the circle. AB and CD are 2 chords which are equidistant from the centre O of the circle, since OB = OD

Now we have to prove OB = OD

Proof:

In ΔMBO and ΔNDO

∟BMO = ∟DNO = 90˚

∟BMO = ∟DNO = 90˚

MO = NO (data given)

OB = OD (radii)

ΔMBO = ΔNDO (RHS)

MB = DN

Hence, AB = CD


  1. Suppose two chords of a circle are unequal in length. Prove that the chord of larger length is nearer to the centre than the chord of smaller length

Solution:

Circles – Exercise 4.4.2 – Class IX

In the figure, O is the center of the circle, where AB and CD are chords. Ab is larger and CD is smaller chord. OP and OQ the distance from the chord AB and CD respectively.

To prove: OP < OQ

Proof:

In ΔAPO;  AQ2 = AP2 + PQ2 …………(1)

In ΔCQO; OC2 = CQ2 + OQ2 ………….(2)

AO = OC radius of the circle.

From (1) and (2),

AP2 + PO2 = CQ2 + OQ2

Let AP = CQ + x                 [AB < CQ; 1/2 AB > 1/2 CD ; AP > CQ]

(CO+X)2 + PO2 = CQ2 + OQ2

CQ2 + 2. CQ. X + PO2 = CQ2 + OQ2

OQ2 = OP2 + 2.CQ.X

or

OQ2 > OP2

OQ > OP

OP < OQ


  1. Let AB and CD be parallel chords of a circle with center O. M is the midpoint of AB and N is the midpoint of CD . Prove that O, N ,M are collinear. If MN = 3cm , AB= 4cm, CD = 10 cm find the radius of the circle.

Solution:

Circles – Exercise 4.4.2 – Class IX

Given: AB and CD are to parallel chords. M and N are midpoint of AB and CD respectively.

To prove: O, N, M are collinear

To find: radius of the circle i.e, OD

Proof:

∟OMB = 90˚ (M is the mid point)

∟ONB = 90˚ and ∟MND = 90˚

∟OND + ∟MND = 180˚

Therefore, O, N , M are collinear.

To find the radius of the circle with centre O.

Let ON = x, in ΔOMB

OB2 = OM2 + MB2

= (3 + x)2 + 22 (AB = 4cm and MB = 2 cm)

= 9 + 6x + x2 + 4

OB2 = x2 + 6x + 13 ……(1)

In ΔOND, OD2 = ON2 + ND2

=  x2 + 52 (CD = 10cm and ND = 5 cm)

OD2 = x2 + 25

OB2 = x2 + 25 …………(2) (OB = OD)[CD = 10cm and ND = 5cm)

x2 + 6x + 13 = x2+ 52

6x = 25 – 13 = 12

x = 12/6 = 2

Thus, x = 2 cm

OD2 = x2 + 52

OD2 = 22 + 52

= 4 + 25 = 29

OD = √29


  1. Circles – Exercise 4.4.2 – Class IXIn the figure ,ABCD is a straight line two concentric circles with centre O. Is it possible to have CD = 6cm, OD = 9cm and OB = 5 cm. Justify.

 

 

 

Solution:

Let us solve the problem using the Pythagoras theorem.

In the figure it is given that OD = 9 cm, OB = 5cm and CD = 6 cm.

Constrction: Draw a line OP from O perpendicular to AD, which cuts the chord AD at P.

Therefore, we have ΔOPD where ∟OPD = 90 ˚, Thus, ΔOPD is a right angled triangle.

By Pythagoras theorem, OD2 = OP2+ PD2

We know OD = 9 thus, 92= OP2+ PD2, where 92 is not a Pythagorian Number.

Therefore,  CD = 6cm, OD = 9cm and OB = 5 cm is not possible.


  1. Circles – Exercise 4.4.2 – Class IX

In the fig A, C, B are collinear centres of three circles and AC =CB. Prove that PQ = RS where the line PQRS passes through the point of intersection Q and R and cuts other two circles at P and Q.

[Hint: draw perpendiculars from A, C, B to PQ]

Solution:

StatementReason
AC = CBGiven

 

Draw AD, CE and BF perpendicular to PQ

 

Construction
AD||CE||BFcorresponding angles are equal i.e., 90˚
DE = EFequal intercepts
DQ + QE = ER + RF

 

⊥ from the centre bisects the chord
1/2PQ + QE = ER + 1/2RS

 

QE = ER
1/2PQ = 1/2RS

 

 
PQ = RS

 

 

  1. Suppose AB and CD are two parallel chords in a circle .prove that the line joining this midpoint passes through the centre of the circle.

Solution:

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Given : O center of the circle and AB ||CD

Construction : Join OM and ON

Proof: M is the midpoint of CD

∟OMB =90 ˚

Similarly N is midpoint of CD

∟OND =90 ˚

∟OMB + ∟BMN = 180 ˚ (∟BMN =90 ˚)

i.e OMN is a straight line

MN passes through O.


  1. Prove that a parallelogram inscribed in a circle is a rectangle

Solution:

Circles – Exercise 4.4.2 – Class IX

Parallelogram ABCD  is cyclic

∟A + ∟C = 180 ˚ [opposite  angle of cyclic quadrilateral]

But ∟A = ∟C = 90 ˚ ……….(1) [opposite  angle of parallelogram are equal]

Similarly, ∟B + ∟D = 180˚

∟B = ∟D = 90˚ ………………..(2) [opposite  angle of parallelogram are equal]

AB = CD and AD = BC ………..(3) [Opposite sides of the  parallelogram]

Opposite sides are equal and each angle is right angle therefore, ABCD is rectangle.