Class 10 - Maths - Constructions

Exercise 11.1

Question 1:

Draw a line segment of length 7.6 cm and divide it in the ratio 5 : 8. Measure the two parts. Give the justification of the construction.

A line segment of length 7.6 cm can be divided in the ratio of 5 : 8 as follows:

Step 1. Draw line segment AB of 7.6 cm and draw a ray AX making an acute angle with line

segment AB.

Step 2. Locate 13 (= 5 + 8) points, A1, A2, A3, A4 ,........, A13 on AX such that AA1 = A1 A2 = A2 A3

and so on.

Step 3. Join BA13.

Step 4. Through the point A5, draw a line parallel to BA13 (by making an angle equal to ∠AA13 B)

at A5 intersecting AB at point C.

Now, C is the point dividing line segment AB of 7.6 cm in the required ratio of 5 : 8.

The lengths of AC and CB can be measured. It comes out to 2.9 cm and 4.7 cm respectively.

Justification:

The construction can be justified by proving that

AC/CB = 5/8

By construction, we have A5 C || A13 B. By applying Basic proportionality theorem for the

triangle AA13 B, we obtain

AC/BC = AA5 /A5 A13 …………... (1)

From the figure, it can be observed that AA5 and A5A13 contain 5 and 8 equal divisions of line

segments respectively.

AA5 /A5 A13 = 5/8  ……….... (2)

On comparing equations (1) and (2), we obtain

AC/CB = 5/8

This justifies the construction.

Question 2:

Construct a triangle of sides 4 cm, 5cm and 6cm and then a triangle similar to it whose sides are 2/3 of the corresponding sides of the first triangle.

Give the justification of the construction.

Step 1. Draw a line segment AB = 4 cm. Taking point A as centre, draw an arc of 5 cm radius.

Similarly, taking point B as its centre, draw an arc of 6 cm radius. These arcs will intersect each

other at point C. Now, AC = 5 cm and BC = 6 cm and ∆ABC is the required triangle.

Step 2. Draw a ray AX making an acute angle with line AB on the opposite side of vertex C.

Step 3. Locate 3 points A1, A2, A3 (as 3 is greater between 2 and 3) on line AX such that

A A1 = AA2 = AA3

Step 4. Join BA3 and draw a line through A2 parallel to BA3 to intersect AB at point B'.

Step 5. Draw a line through B' parallel to the line BC to intersect AC at C'.

Now, ∆AB'C' is the required triangle.

Justification:

The construction can be justified by proving that

AB’ = 2AB/3, B’C’ = 2BC/3, AC’ = 2AC/3

By construction, we have B’C’ || BC

So, ∠A = ∠ABC           (Corresponding angles)

In ∆AB'C' and ∆ABC,

∠ AB’C’ = ∠ABC          (Proved above)

∠ B’AC’ = ∠BAC          (Common)

So, ∆ AB’C’ ∼ ∆ABC       (AA similarity criterion)

=> AB’/AB = BC’/BC = AC’/AC  …….....1

In ∆AA2 B' and ∆AA3 B,

∠A2 AB' = ∠A3 AB           (Common)

∠AA2 B' = ∠AA3 B           (Corresponding angles)

So, ∆AA2 B' ∼ ∆AA3 B     (AA similarity criterion)

=> AB’/AB = AA2/AA3

=> AB’/AB = 2/3   ………..2

From equations 1 and 2, we obtain

AB’/AB = B’C’/BC = AC’/AC = 2/3

=> AB’ = 2AB/3, B’C’ = 2BC/3, AC’ = 2AC/3

This justifies the construction.

Question 3:

Construct a triangle with sides 5 cm, 6 cm and 7 cm and then another triangle whose sides are 7/5 of the corresponding sides of the first triangle.

Give the justification of the construction.

Step 1. Draw a line segment AB of 5 cm. Taking A and B as centre, draw arcs of 6 cm and 5 cm

radius respectively. Let these arcs intersect each other at point C. ∆ABC is the required triangle

having length of sides as 5 cm, 6 cm, and 7 cm respectively.

Step 2. Draw a ray AX making acute angle with line AB on the opposite side of vertex C.

Step 3. Locate 7 points, A1, A2, A3, A4 , A5, A6, A7 (as 7 is greater between 5and 7), on

line AX such that AA1 = A1 A2 = A2 A3 = A3 A4 = A4 A5 = A5 A6 = A6 A7.

Step 4. Join B5 and draw a line through A7 parallel to BA5 to intersect extended line segment AB

at point B'.

Step 5. Draw a line through B' parallel to BC intersecting the extended line segment AC at C'.

Now, ∆AB'C' is the required triangle.

Justification:

The construction can be justified by proving that

AB’ = 7AB/5, B’C’ = 7BC/5, AC’ = 7AC/5

In ∆ABC and ∆AB'C',

∠ABC = ∠AB'C' (Corresponding angles)

∠BAC = ∠B'AC' (Common)

So, ∆ABC ∼ ∆AB'C' (AA similarity criterion)

=> AB/AB’ = BC/B’C’ = AC/AC’  …….... (1)

In ∆AA5 B and ∆AA7 B',

∠A5 AB = ∠A7 AB' (Common)

∠AA5 B = ∠AA7 B' (Corresponding angles)

So, ∆AA5 B ∼ ∆AA7 B' (AA similarity criterion)

=> AB/AB’ = AA5 /AA7

=> AB/AB’ = 5/7   ………….(2)

On comparing equations (1) and (2), we obtain

AB/AB’ = BC/B’C’ = AC/AC’ = 5/7

AB’ = 7AB/5, B’C’ = 7BC/5, AC’ = 7AC/5

This justifies the construction.

Question 4:

Construct an isosceles triangle whose base is 8 cm and altitude 4 cm and then another triangle whose side are

1½ times the corresponding sides of the isosceles triangle. Give the justification of the construction.

Let us assume that ∆ABC is an isosceles triangle having CA and CB of equal lengths, base AB of

8 cm, and AD is the altitude of 4 cm.

The ∆AB'C' whose sides are 3/2 times of ∆ABC can be drawn as follows:

Step 1. Draw a line segment AB of 8 cm. Draw arcs of same radius on both sides of the line

segment while taking point A and B as its centre. Let these arcs intersect each other at O and

O'. Join OO'. Let OO' intersect AB at D.

Step 2. Taking D as centre, draw an arc of 4 cm radius which cuts the extended line segment

OO' at point C. An isosceles ∆ABC is formed, having CD (altitude) as 4 cm and AB (base) as 8

cm.

Step 3. Draw a ray AX making an acute angle with line segment AB on the opposite side of

vertex C.

Step 4. Locate 3 points (as 3 is greater between 3 and 2) A1, A2, and A3 on AX such that

AA1 = A1 A2 = A2 A3.

Step 5. Join BA2 and draw a line through A3 parallel to BA2 to intersect extended line segment

AB at point B'.

Step 6. Draw a line through B' parallel to BC intersecting the extended line segment AC at C'.

Now, ∆AB'C' is the required triangle.

Justification:

The construction can be justified by proving that

AB’ = 3AB/2, B’C’ = 3BC/2, AC’ = 3AC/2

In ∆ABC and ∆AB'C',

∠ABC = ∠AB'C' (Corresponding angles)

∠BAC = ∠B'AC' (Common)

So, ∆ABC ∼ ∆AB'C' (AA similarity criterion)

=> AB/AB’ = BC/B’C’= AC/AC’  …..... (1)

In ∆AA2 B and ∆AA3 B',

∠A2 AB = ∠A3 AB' (Common)

∠AA2 B = ∠AA3 B' (Corresponding angles)

So, ∆AA2 B ∼ ∆AA3 B' (AA similarity criterion)

=> AB/AB’ = AA2 /AA3

=> AB/AB’ = 2/3  ………..(2)

On comparing equations (1) and (2), we obtain

AB/AB’ = BC/B’C’ = AC/AC’ = 2/3

AB’ = 3AB/2, B’C’ = 3BC/2, AC’ = 3AC/2

This justifies the construction.

Question 5:

Draw a triangle ABC with side BC = 6 cm, AB = 5 cm and ∠ABC = 60°.

Then construct a triangle whose sides are 3/4 of the corresponding sides of the triangle ABC. Give the justification of the construction.

A ∆A'BC' whose sides are 3/4 of the corresponding sides of ∆ABC can be drawn as follows.

Step 1. Draw a ∆ABC with side BC = 6 cm, AB = 5 cm and ∠ABC = 60°.

Step 2. Draw a ray BX making an acute angle with BC on the opposite side of vertex A.

Step 3. Locate 4 points (as 4 is greater in 3 and 4), B1, B2, B3, B4, on line segment BX.

Step 4. Join B4 C and draw a line through B3, parallel to B4C intersecting BC at C'.

Step 5. Draw a line through C' parallel to AC intersecting AB at A'. ∆A'BC' is the required

triangle.

Justification:

The construction can be justified by proving

A’B = 3AB/4, BC’ = 3BC/4, A’C’ = 3AC/4

In ∆A'BC' and ∆ABC,

∠A'C'B = ∠ACB          (Corresponding angles)

∠A'BC' = ∠ABC          (Common)

So, ∆A'BC' ∼ ∆ABC    (AA similarity criterion)

A’B/AB = BC’/BC = A’C’/AC  …….... (1)

In ∆BB3 C' and ∆BB4 C,

∠B3 BC' = ∠B4 BC            (Common)

∠BB3 C' = ∠BB4 C            (Corresponding angles)

So, ∆BB3 C' ∼ ∆BB4 C     (AA similarity criterion)

BC’/BC = BB3/AA4

=> BC’/BC = 3/4      ………….(2)

From equations (1) and (2), we obtain

=> A’B/AB = BC’/BC = A’C’/AC = 3/4

=> A’B = 3AB/4, BC’ = 3BC/4, A’C’ = 3AC/4

This justifies the construction.

Question 6:

Draw a triangle ABC with side BC = 7 cm, ∠B = 450, ∠A = 1050. Then, construct a triangle whose sides are 4/3 times the corresponding side of ∆ABC.

Give the justification of the construction.

Given, ∠B = 450, ∠A = 1050

Sum of all interior angles in a triangle is 1800.

∠A + ∠B + ∠C = 1800

=> 1050 + 450 + ∠C = 1800

=> ∠C = 1800 − 1500

=> ∠C = 300

The required triangle can be drawn as follows:

Step 1. Draw a ∆ABC with side BC = 7 cm, ∠B = 450, ∠C = 300.

Step 2. Draw a ray BX making an acute angle with BC on the opposite side of vertex A.

Step 3. Locate 4 points (as 4 is greater in 4 and 3), B1, B2, B3, B4, on BX.

Step 4. Join B3 C. Draw a line through B4 parallel to B3 C intersecting extended BC at C'.

Step 5. Through C', draw a line parallel to AC intersecting extended line segment at C'.

Now, ∆A'BC' is the required triangle.

Justification:

The construction can be justified by proving that

A’B = 4AB/3, BC’ = 4BC/3, A’C’ = 4AC/3

In ∆ABC and ∆A'BC',

∠ABC = ∠A'BC' (Common)

∠ACB = ∠A'C'B (Corresponding angles)

So, ∆ABC ∼ ∆A'BC' (AA similarity criterion)

AB/A’B = BC/BC’ = AC/A’C’   ……... (1)

In ∆BB3 C and ∆BB4 C',

∠B3 BC = ∠B4 BC' (Common)

∠BB3 C = ∠BB4 C' (Corresponding angles)

So, ∆BB3 C ∼ ∆BB4 C' (AA similarity criterion)

BC/BC’ = BB3 /BB4

=> BC/BC’ = 3/4 ………….(2)

On comparing equations (1) and (2), we obtain

=> AB/A’B = BC/BC’ = AC/A’C’ = 3/4

=> A’B = 4AB/3, BC’ = 4BC/3, A’C’ = 4AC/3

This justifies the construction.

Question 7:

Draw a right triangle in which the sides (other than hypotenuse) are of lengths 4 cm and 3 cm.

the construct another triangle whose sides are 5/3 times the corresponding sides of the given triangle. Give the justification of the construction.

It is given that sides other than hypotenuse are of lengths 4 cm and 3 cm. Clearly, these will be

perpendicular to each other. The required triangle can be drawn as follows:

Step 1. Draw a line segment AB = 4 cm. Draw a ray SA making 900 with it.

Step 2. Draw an arc of 3 cm radius while taking A as its centre to intersect SA at C. Join BC.

So, ∆ABC is the required triangle.

Step 3. Draw a ray AX making an acute angle with AB, opposite to vertex C.

Step 4. Locate 5 points (as 5 is greater in 5 and 3), A1, A2, A3, A4, A5, on line segment AX such

that AA1 = A1 A2 = A2 A3 = A3 A4 = A4 A5.

Step 5. Join A3 B. Draw a line through A5 parallel to A3 B intersecting extended line segment AB

at B'.

Step 6. Through B', draw a line parallel to BC intersecting extended line segment AC at C'.

So, ∆AB'C' is the required triangle.

Justification:

The construction can be justified by proving that

AB’ = 5AB/3, B’C’ = 5BC/3, AC’ = 5AC/3

In ∆ABC and ∆AB'C',

∠ABC = ∠AB'C' (Corresponding angles)

∠BAC = ∠B'AC' (Common)

So, ∆ABC ∼ ∆AB'C' (AA similarity criterion)

AB/AB’ = BC/B’C’ = AC/AC’  ……….... (1)

In ∆AA3 B and ∆AA5 B',

∠A3 AB = ∠A5 AB' (Common)

∠AA3 B = ∠AA5 B' (Corresponding angles)

So, ∆AA3 B ∼ ∆AA5 B' (AA similarity criterion)

=> AB/AB’ = AA3 /AA5

=> AB/AB’ = 3/5  ………..(2)

On comparing equations (1) and (2), we obtain

AB/AB’ = BC/B’C’ = AC/AC’ = 3/5

AB’ = 5AB/3, B’C’ = 5BC/3, AC’ = 5AC/3

This justifies the construction.

Exercise 11.2

Question 1:

Draw a circle of radius 6 cm. From a point 10 cm away from its centre, construct the pair of tangents to the circle and measure their lengths.

Give the justification of the construction.

Answer: A pair of tangents to the given circle can be constructed as follows:

Step 1. Taking any point O of the given plane as centre, draw a circle of 6 cm radius. Locate a

point P, 10 cm away from O. Join OP.

Step 2. Bisect OP. Let M be the mid-point of PO.

Step 3. Taking M as centre and MO as radius, draw a circle.

Step 4. Let this circle intersect the previous circle at point Q and R.

Step 5. Join PQ and PR. PQ and PR are the required tangents.

The lengths of tangents PQ and PR are 8 cm each.

Justification:

The construction can be justified by proving that PQ and PR are the tangents to the circle

(whose centre is O and radius is 6 cm). For this, join OQ and OR.

∠PQO is an angle in the semi-circle. We know that angle in a semi-circle is a right angle.

So, ∠PQO = 900

=> OQ ⊥ PQ

Since OQ is the radius of the circle, PQ has to be a tangent of the circle.

Similarly, PR is a tangent of the circle.

Question 2:

Construct a tangent to a circle of radius 4 cm from a point on the concentric circle of radius 6 cm and measure its length.

Also verify the measurement by actual calculation. Give the justification of the construction.

Tangents on the given circle can be drawn as follows:

Step 1. Draw a circle of 4 cm radius with centre as O on the given plane.

Step 2. Draw a circle of 6 cm radius taking O as its centre. Locate a point P on this circle and

join OP.

Step 3. Bisect OP. Let M be the mid-point of PO.

Step 4. Taking M as its centre and MO as its radius, draw a circle. Let it intersect the given

circle at the points Q and R.

Step 5. Join PQ and PR. PQ and PR are the required tangents.

It can be observed that PQ and PR are of length 4.47 cm each.

In ∆PQO,

Since PQ is a tangent,

∠PQO = 900

PO = 6 cm

QO = 4 cm

Applying Pythagoras theorem in ∆PQO, we obtain

PQ2 + QO2 = PQ2

=> PQ2 + 42 = 62

=> PQ2 + 16 = 36

=> PQ2 = 36 − 16

=> PQ2 = 20

=> PQ = √20

=> PQ = 4.47 cm

Justification:

The construction can be justified by proving that PQ and PR are the tangents to the circle

(whose centre is O and radius is 4 cm). For this, let us join OQ and OR.

∠PQO is an angle in the semi-circle. We know that angle in a semi-circle is a right angle.

So, ∠PQO = 900

=> OQ ⊥ PQ

Since OQ is the radius of the circle, PQ has to be a tangent of the circle.

Similarly, PR is a tangent of the circle.

Question 3:

Draw a circle of radius 3 cm. Take two points P and Q on one of its extended diameter each at a distance of 7 cm from its centre.

Draw tangents to the circle from these two points P and Q. Give the justification of the construction.

The tangent can be constructed on the given circle as follows:

Step 1. Taking any point O on the given plane as centre, draw a circle of 3 cm radius.

Step 2. Take one of its diameters, PQ, and extend it on both sides. Locate two points on this

diameter such that OR = OS = 7 cm.

Step 3. Bisect OR and OS. Let T and U be the mid-points of OR and OS respectively.

Step 4. Taking T and U as its centre and with TO and UO as radius, draw two circles. These two

circles will intersect the circle at point V, W, X, Y respectively. Join RV, RW, SX, and SY.

These are the required tangents.

Justification:

The construction can be justified by proving that RV, RW, SY, and SX are the tangents to the

circle (whose centre is O and radius is 3 cm). For this, join OV, OW, OX, and OY.

∠RVO is an angle in the semi-circle. We know that angle in a semi-circle is a right angle.

So, ∠RVO = 900

=> OV ⊥ RV

Since OV is the radius of the circle, RV has to be a tangent of the circle.

Similarly, OW, OX, and OY are the tangents of the circle.

Question 4:

Draw a pair of tangents to a circle of radius 5 cm which are inclined to each other at an angle of 600. Give the justification of the construction.

The tangents can be constructed in the following manner:

Step 1. Draw a circle of radius 5 cm and with centre as O.

Step 2. Take a point A on the circumference of the circle and join OA. Draw a perpendicular to

OA at point A.

Step 3. Draw a radius OB, making an angle of 1200 (1800 − 600) with OA.

Step 4. Draw a perpendicular to OB at point B. Let both the perpendiculars intersect at point P.

PA and PB are the required tangents at an angle of 600.

Justification:

The construction can be justified by proving that ∠APB = 600

By our construction

∠OAP = 900

∠OBP = 900

and ∠AOB = 1200

We know that the sum of all interior angles of a quadrilateral = 3600

∠OAP + ∠AOB + ∠OBP + ∠APB = 3600

=> 900 + 1200 + 900 + ∠APB = 3600

=> 3000 + ∠APB = 3600

=> ∠APB = 3600 - 3000

=> ∠APB = 600

This justifies the construction.

Question 5:

Draw a line segment AB of length 8 cm. Taking A as centre, draw a circle of radius 4 cm and taking B as centre,

draw another circle of radius 3 cm. Construct tangents to each circle from the centre of the other circle. Give the justification of the construction.

The tangents can be constructed on the given circles as follows:

Step 1. Draw a line segment AB of 8 cm. Taking A and B as centre, draw two circles of 4 cm and

Step 2. Bisect the line AB. Let the mid-point of AB be C. Taking C as centre, draw a circle of AC

radius which will intersect the circles at points P, Q, R, and S. Join BP, BQ, AS, and AR.

These are the required tangents.

Justification:

The construction can be justified by proving that AS and AR are the tangents of the circle

(whose centre is B and radius is 3 cm) and BP and BQ are the tangents of the circle (whose

centre is A and radius is 4 cm). For this, join AP, AQ, BS, and BR.

∠ASB is an angle in the semi-circle. We know that an angle in a semi-circle is a right angle.

So, ∠ASB = 900

=> BS ⊥ AS

Since BS is the radius of the circle, AS has to be a tangent of the circle.

Similarly, AR, BP, and BQ are the tangents.

Question 6:

Let ABC be a right triangle in which AB = 6 cm, BC = 8 cm and ∠B = 900. BD is the perpendicular from B on AC.

The circle through B, C, and D is drawn. Construct the tangents from A to this circle. Give the justification of the construction.

Consider the following situation. If a circle is drawn through B, D, and C, BC will be its diameter

as ∠BDC is of measure 900. The centre E of this circle will be the mid-point of BC.

The required tangents can be constructed on the given circle as follows:

Step 1. Join AE and bisect it. Let F be the mid-point of AE.

Step 2. Taking F as centre and FE as its radius, draw a circle which will intersect the circle at

point B and G. Join AG.

AB and AG are the required tangents.

Justification:

The construction can be justified by proving that AG and AB are the tangents to the circle. For

this, join EG.

∠AGE is an angle in the semi-circle. We know that an angle in a semi-circle is a right angle.

So, ∠AGE = 900

=> EG ⊥ AG

Since EG is the radius of the circle, AG has to be a tangent of the circle.

Again, ∠B = 900

=> AB ⊥ BE

Since BE is the radius of the circle, AB has to be a tangent of the circle.

Question 7:

Draw a circle with the help of a bangle. Take a point outside the circle.

Construct the pair of tangents from this point to the circles. Give the justification of the construction.

The required tangents can be constructed on the given circle as follows:

Step 1. Draw a circle with the help of a bangle.

Step 2. Take a point P outside this circle and take two chords QR and ST.

Step 3. Draw perpendicular bisectors of these chords. Let them intersect each other at point O.

Step 4. Join PO and bisect it. Let U be the mid-point of PO. Taking U as centre, draw a circle of

radius OU, which will intersect the circle at V and W. Join PV and PW.

PV and PW are the required tangents.

Justification:

The construction can be justified by proving that PV and PW are the tangents to the circle. For

this, first of all, it has to be proved that O is the centre of the circle. Let us join OV and OW.

We know that perpendicular bisector of a chord passes through the centre. Therefore, the

perpendicular bisector of chords QR and ST pass through the centre. It is clear that the

intersection point of these perpendicular bisectors is the centre of the circle. ∠PVO is an angle

in the semi-circle. We know that an angle in a semi-circle is a right angle.

So, ∠PVO = 900

=> OV ⊥ PV

Since OV is the radius of the circle, PV has to be a tangent of the circle.

Similarly, PW is a tangent of the circle.