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Page No 55:

Question 1:

Write the correct answer in each of the following:
In the figure, if AB ∥ CD ∥ EF, PQ ∥ RS, ∠RQD = 25° and ∠CQP = 60°, then ∠QRS is equal to

(A) 85°
(B) 135°
(C) 145°
(D) 110°

Answer:

As PQRS, produce PQ to M.
CQP=MQD              Vertically opppsite angles
60°=1+25°1=35°


Also,
2=MQD              Corresponding angles2=60°

Since, PMRS.
2+SRM=180°SRM=180°-60°=120°

Now,
2+RMQ=180°          linear pairRMQ=180°-60°=120°

Applying angle sum property in MQR,
1+3+QMR=180°35°+3+120°=180°3=25°

Finally,
QRS=ARQ+ARS=25°+120°=145°

Hence, the correct answer is option C.

Page No 55:

Question 2:

Write the correct answer in each of the following:
If one angle of a triangle is equal to the sum of the other two angles, then the triangle is
(A) an isosceles triangle
(B) an obtuse triangle
(C) an equilateral triangle
(D) a right triangle

Answer:

Take a ABC with the angles being A, B and C.

Given that,
A=B+C                .....1In ABC, by angle sum property of triangles,A+B+C=180°2A=180°            using 1A=90°

Thus, the triangle is a right angled triangle.
Hence, the correct answer is option D.

Page No 55:

Question 3:

Write the correct answer in each of the following:
An exterior angle of a triangle is 105° and its two interior opposite angles are equal. Each of these equal angles is

(A) 371°2

(B) 521°2

(C) 721°2

(D) 75°

Answer:

Consider each of the two interior opposite angle be x.
An exterior angle of the given triangle is 105°. 
The exterior angle of a triangle is equal to the sum of the two interior opposite angles.

105°=x+x2x=105°x=12×105°=521°2
So, each angle is 52.5°.
Hence, the correct answer is option B.

Page No 55:

Question 4:

Write the correct answer in each of the following:
The angles of a triangle are in the ratio 5 : 3 : 7. The triangle is
(A) an acute angled triangle
(B) an obtuse angled triangle
(C) a right triangle
(D) an isosceles triangle

Answer:

By angle sum property of triangles, the sum of the angles of the triangle is 180°.
Thus, let the angle of triangle be 5x, 3x and 7x.
5x+3x+7x=180°15x=180°x=180°15x=12°

∴ The angle measures of the triangle are:
5×12°, 3×12° and 7×12°i.e. 60°, 36° and 84°
As, the measure of each angle of triangle is less than 90°, so the angles of the triangle are acute angles.
Hence, the correct answer is option A.

Page No 55:

Question 5:

Write the correct answer in each of the following:
If one of the angles of a triangle is 130°, then the angle between the bisectors of the other two angles can be
(A) 50°
(B) 65°
(C) 145°
(D) 155°

Answer:

Consider PQR.
Given that, P=130°.



Now, OQ and OR are the bisectors of the angles Q and R.
QOR=180°-12PQR+PRQ=180°-12×50°=180°-25°=155°
Hence, the correct answer is option D.

Page No 55:

Question 6:

Write the correct answer in each of the following:
In the figure, POQ is a line. The value of x is

(A) 20°
(B) 25°
(C) 30°
(D) 35°

Answer:

Given that,
3x+4x+40°=180°              linear pair7x+40°=180°7x=140°x=20°
Thus, the value of x is 20°.
Hence, the correct answer is option A.



Page No 56:

Question 7:

Write the correct answer in each of the following:
In the figure, if OP ∥ RS, ∠OPQ = 110° and ∠QRS = 130°, then ∠PQR is equal to

(A) 40°
(B) 50°
(C) 60°
(D) 70°

Answer:

In the figure given, producing OP, intersects RQ at X.
Since OP RS and RX is the transversal.
So,
RXP=XRSRXP=130°        .....1   QRS=130°

Now, RQ is a line segment.
So, PXQ+RXP=180°          Linear pair axiom
PXQ=180°-RXP               =180°-130°PXQ=50°

In PQX, OPQ is an exterior angle.
OPQ=PXQ+PQX             [exterior angle = sum of 2 opposite interior angle]
110°=50°+PQXPQX=110°-50°PQX=60°
PQR=60°             PQR=PQX
Hence, the correct answer is option C.

Page No 56:

Question 8:

Write the correct answer in each of the following:
Angles of a triangle are in the ratio 2 : 4 : 3. The smallest angle of the triangle is
(A) 60°
(B) 40°
(C) 80°
(D) 20°

Answer:

The ratio of angles of a triangle is 2 : 4 : 3.
Let the angles of the triangle be A, B and C.
A=2x, B=4x and C=3xIn ABC,A+B+C=180°        sum of angle of triangle is 180°
2x+4x+3x=180°9x=180°x=1809=20°A=2x=2×20°=40°B=4x=4×20°=80°and  C=3x=3×20°=60°
Thus, the smallest angle of a triangle is 40°.
Hence, the correct answer is option B.

Page No 56:

Question 1:

For what value of x + y In the Figure will ABC be a line? Justify your answer.

Answer:

In the given figure, x and y are two adjacent angles.
For ABC to be a straight line, the sum of two adjacent angles x and y must be 180°.

Page No 56:

Question 2:

Can a triangle have all angles less than 60°? Give reason for your answer.

Answer:

A triangle cannot have all angles less than 60°. This is because the sum of the angles of the triangle will be less than 180°, which does not satisfy the angle sum property of triangles.

Page No 56:

Question 3:

Can a triangle have two obtuse angles? Give reason for your answer.

Answer:

According to angle sum property of triangles, the sum of the three angles of a triangle is 180°.

Thus, a triangle cannot have 2 obtuse-angles because the sum of all the angles cannot exceed 180°.

Page No 56:

Question 4:

How many triangles can be drawn having its angles as 45°, 64° and 72°? Give reason for your answer.

Answer:

As, 45°+64°+72°=181°180°.
We cannot draw any triangle having its angle as 45°, 64° and 72° because the sum of the angles cannot be 181°.
 



Page No 57:

Question 5:

How many triangles can be drawn having its angles as 53°, 64° and 63°? Give reason for your answer.

Answer:

As, sum of the angles = 53°+64°+63°=180°.
Thus, we can draw infinitely many triangles, whose sum of the angles of every triangle, having angles as 53°, 64° and 63° is 180°.

Page No 57:

Question 6:

In the figure, find the value of x for which the lines l and m are parallel.

Answer:

If a transversal intersects two parallel lines, then each pair of consecutive interior angles are supplementary. 

Here, the two given lines l and m are parallel.
Now, the angles x° and 44° are consecutive interior angles on the same side of the transversal.
 x+44°=180°x=180°-44°x=136°
 

Page No 57:

Question 7:

Two adjacent angles are equal. Is it necessary that each of these angles will be a right angle? Justify your answer.

Answer:

No, each of these angles will be a right angle only when they form a linear pair. i.e. when the non-common arms of the given two adjacent angle are two opposite rays.

Page No 57:

Question 8:

If one of the angles formed by two intersecting lines is a right angle, what can you say about the other three angles? Give reason for your answer.

Answer:

If two line intersect each other at a point, then four angles are formed. If one of these four angles is a right angle, then each of other three angle will be a right angle by linear pair axiom.

Page No 57:

Question 9:

In the figure, which of the two lines are parallel and why?

Answer:

For figure (1), a transversal intersects two lines such that the sum of interior angles on the same side of the transversal is 
132°+48°=180°
Thus, the lines l and m are parallel.

For figure (2), a transversal intersects two lines such that the sum of interior angles on the same side of the transversal is
73°+106°=179°
Therefore, the line p and q are not parallel.

Page No 57:

Question 10:

Two lines l and m are perpendicular to the same line n. Are l and m perpendicular to each other? Give reason for your answer.

Answer:

When 2 lines l and m are perpendicular to the same line n, each of the two corresponding angles formed by these lines l and m with the line n equal to 90°.
Hence, l and m are parallel to each other.



Page No 58:

Question 1:

In the figure, OD is the bisector of ∠AOC, OE is the bisector of ∠BOC and OD ⊥ OE. Show that the points A, O and B are collinear.

Answer:

In the figure, OD ⏊ OE, OD and OE are the bisector of AOC and BOC respectively.

As, OD and DE bisects AOC and BOC respectively.
AOC=2DOC          .....1COB=2COE               .....2

On adding equations (1) and (2),
AOC +COB=2DOC+2COEAOC+COB=2DOC+COEAOC+COB=2DOEAOC+COB=2×90°              ODOEAOC+COB=180°AOB=180°

So, AOC+COB are forming linear pair i.e. AOB is a straight line.
Hence, point A, O and B are collinear.
 

Page No 58:

Question 2:

In the figure, ∠1 = 60° and ∠6 = 120°. Show that the lines m and n are parallel.

Answer:

From the figure,
5+6=180°              linear pair5+120°=180°5=60°1=5=60°

But, these are corresponding angles. Hence, m and n are parallel.

Page No 58:

Question 3:

AP and BQ are the bisectors of the two alternate interior angles formed by the intersection of a transversal t with parallel lines l and m. Show that AP ∥ BQ.

Answer:

As, lm and t is the transversal.

MAB=ABN         Alternate angles
12MAB=12ABNPAB=ABQ

Now, PAB and ABQ are alternate angles.
Hence, AP ∥ BQ.



Page No 59:

Question 4:

In the figure, bisectors AP and BQ of the alternate interior angles are parallel, then show that l m.

Answer:

In the given figure, it is given that AP ∥ BQ.


Now, AP is the bisector of MAB and BQ is the bisector of ABN.

As AP ∥ BQ,
PAB=ABQ           Alternate angles2PAB=2ABQMAB=ABN
As, these are alternate angles.
Hence, lm.
 

Page No 59:

Question 5:

In the figure, BA ∥ ED and BC  EF. Show that ∠ABC = ∠DEF.

Answer:

Produce DE to intersect BC at P.
EFBC and DP is the transversal.

DEF=DPC                  .....1 Corresponding angles

Now, ABDP and BC is the transversal.
DPC=ABC               .....2 Corresponding angles

From (1) and (2),
ABC=DEF
Hence, proved.

Page No 59:

Question 6:

In the Figure BA || ED and BC || EF. Show that ∠ABC + ∠DEF = 180°

Answer:

Produce ED to meet BC at point P.


Now,
EFBC and EP is the transversal.

The sum of co-interior angles on the same side of the transversal is 180°.
DEF+EPC=180°            .....1

Again, EPAB and BC is transversal.
EPC=ABC              .....2   Corresponding angles

From (1) and (2),
DEF+ABC=180°ABC+DEF=180°
Hence, proved.
 

Page No 59:

Question 7:

In the figure, DE  QR and AP and BP are bisectors of ∠EAB and ∠RBA respectively. Find ∠APB.

Answer:

Given that, DEQR and line n is the transversal line.

The sum of two co-interior angles on the same side of the transversal is 180°.
EAB+RBA=180°            .....112EAB+RBA=12×180°
PAB+PBA=90°        [∵ AP is the bisector of EAB and BP is the bisector of RBA]

Now, in APB,
APB=180°-PAB+PBAAPB=180°-90°APB=90°
Hence, APB=90°.
 



Page No 60:

Question 8:

The angles of a triangle are in the ratio 2 : 3 : 4. Find the angles of the triangle.

Answer:

Given that, the ratio of angles 2 : 3 : 4.

Let angles of triangle be A, B and C.
A=2x,B=3x andC=4x

In ABC,A+B+C=180°        sum of angles of  is 180°2x+3x+4x=180°9x=180°x=20°

Thus,
A=2×10=40°B=3×20=60°C=4×20=80°

Hence, the angles of the ABC are 40°, 60° and 80°.

Page No 60:

Question 9:

A triangle ABC is right angled at A. L is a point on BC such that AL ⊥ BC. Prove that ∠BAL = ∠ACB.

Answer:

In ABC
A=90° and ALBC,

To prove : BAL=ACBBAL=ALC 1 Each 90°and ABC=ABL  2 Commom angleAdding 1 and 2BAC+ABC=ALC+ABC 3
In  ABC,BAC+ACB+ABC=180° sum of angles of triangles is 180°BAC+ABC=180°-ACB  4
In ABL,ABL+ALB+BAL=180°  sum angles of triangle is 180°
ABL+ALC=180°-BAL 5 ALC=ALB=90°
Substituting the value from equation 4 and 5 in equation 3, we get.
180°-ACB=180°-BALACB=BAL.
Hence, proved.

Page No 60:

Question 10:

Two lines are respectively perpendicular to two parallel lines. Show that they are parallel to each other.

Answer:

Two lines p and n are respectively perpendicular to two parallel lines l and  m, i.e., pl and nm.
Show that: p is parallel to n.

As nm,
so, 1=90°       .....1

Again, pl,
so, 2=90°                .....2

But l is parallel to m, so
2=3                      Corresponding angles3=90°          .....2   2=90° 

From (1) and (2),
1=3       Each=90°

As, these are corresponding angles.
Hence, pn



Page No 61:

Question 1:

If two lines intersect, prove that the vertically opposite angles are equal.

Answer:

Given that, the two lines AB and CD intersect at point O.


To prove:
iAOC=BODiiAOD=BOC

Proof:
i Since, ray OA stands on line CD.AOC+AOD=180°               .....1  Linear pair axiom

Similarly, ray OD stands line AB.
AOD+BOD=180°           .....2

From (1) and (2),
AOC+AOD=AOD+BODAOC=BOD

Hence, proved.

(ii) Since, ray OD stands on line AB.
AOD+BOD=180°          .....3

Similarly, ray OB stands on line CD.
DOB+BOC=180°              .....4

From (3) and (4),
AOD+BOD=DOB+BOCAOD=BOC

Hence, proved.

Page No 61:

Question 2:

Bisectors of interior ∠B and exterior ∠ACD of a ABC intersect at the point T. Prove that BTC=12BAC.

Answer:

For a triangle ABC, produce BC to D and the bisectors of ABC and ACD meet at point T.

Now, 
CT is a bisector of ACD12 ACD=TCD 

Also,
BT is a bisector of ABCCBT=12ABC

In ABC, ACD is an exterior angle.
ACD=ABC+CAB
12 ACD=12 CAB+12 ABCTCD=12 CAB+12 ABC      .....1

Again, in BTC,
TCD=BTC+CBT          Exterior angle=sum of two opposite angles
TCD=BTC+12ABC     .....2

From (1) and (2),
12 CAB+12 ABC=BTC+12ABC12CAB=BTC or 12BAC=BTC
Hence, proved.

Page No 61:

Question 3:

A transversal intersects two parallel lines. Prove that the bisectors of any pair of corresponding angles so formed are parallel.

Answer:

Consider AB and CD to be the two parallel lines and are intersected by transversal GH at P and Q.
Now, EP and FQ be the bisectors of corresponding angles APG and CQP.

Since, ABCD.
APG=CQP   Corresponding angles12APG=12CQPEPG=FQP        EP and FQ are bisectors
As, the corresponding angle made by the lines EP and FQ and the transversal line GH are EPG and FQP and they are equal.
Hence, EPFQ.



Page No 62:

Question 4:

Prove that through a given point, we can draw only one perpendicular to a given line.

Answer:

Consider three lines l, m and n. A point P formed by intersection of the lines m and n.

The lines m and n are intersecting at P and are perpendicular to line l.

In APB,A+P+B=180°              Angle sum property of triangles90°+P+90°=180°P+180°=180°P=0°
i.e., the two lines m and n coincide.

Hence, only one perpendicular line can be drawn through a given point on a given line.
 

Page No 62:

Question 5:

Prove that two lines that are respectively perpendicular to two intersecting lines intersect each other.

Answer:

Consider two lines l and m to be intersecting.
Also, consider another two lines n and p perpendicular to m and l respectively.

Assume that the lines n and p are not intersecting. This means they will be parallel to each other.
np       .....1

Since, n and p are perpendicular to m and l respectively.
nm and pl

But from (1), lm which shows contradiction.
Thus, our assumption was wrong.
Hence, the lines n and p intersect at a point D.

Page No 62:

Question 6:

Prove that a triangle must have atleast two acute angles.

Answer:

Consider ABC. By angle sum property of triangles,
A+B+C=180°          .....1

Case 1: When two angles are 90°.
Let ∠B and ∠C be 90° in measure.

Thus, from (1),
A+90°+90°=180°A+180°=180°
A=0
Thus, triangle is not possible.

Case 2: When two angles are obtuse.
Let ∠B and ∠C be obtuse angles.

Thus, from (1),
A=180°-B+CA=180°-sum greater than 180°A=negative angle, which is not possible
Thus, triangle is not possible.

Case 3: When one angle is 90°.
Let ∠B = 90°.

Therefore,
A+B+C=180°A+C+90°=180°A+C=90°
Thus, sum of the other two angles are 90°.
Hence, both angles are acute.

Case 4: When two angles are acute.
Then, the sum of two angles < 180°, then the third angle may be an acute or obtuse angle.
Thus, a triangle should have at least 2 acute angles.

Page No 62:

Question 7:

In the figure, ∠Q > ∠R, PA is the bisector of ∠QPR and PM ⊥ QR. Prove that APM=12Q-R.

Answer:

As, PA is bisector of QPR.
QPA=APR      .....1

In PQM, by angle sum property of triangles,
PQM+PMQ+QPM=180°PQM+90°+QPM=180°PQM=90°-QPMQ=90°-QPM                     .....2

Again, in PMR, by angle sum property of triangles,
PMR+PRM+RPM=180°90°+PRM+RPM=180°PRM=90°-RPMR=90°-RPM               .....3

Subtracting (3) from (2),
Q-R=90°-QPM-90°-RPM
Q-R=RPM-QPM=RPA+APM-QPA-APM=RPA+APM-QPA+APM=2APM          From (1)
APM=12 Q-R



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