Class 7 Math NCERT Solutions for Chapter – 10 Practical Geometry Ex – 10.4

Practical Geometry

1. Construct ΔABC, given m ∠A =60o, m ∠B = 30o and AB = 5.8 cm.

Solution:-

Steps of construction:

1. Draw a line segment AB = 5.8 cm.

2. At point A, draw a ray P to making an angle of 60o i.e. ∠PAB = 60o.

3. At point B, draw a ray Q to making an angle of 30o i.e. ∠QBA = 30o.

4. Now the two rays AP and BQ intersect at the point C.

Then, ΔABC is the required triangle.

2. Construct ΔPQR if PQ = 5 cm, m∠PQR = 105o and m∠QRP = 40o.

(Hint: Recall angle-sum property of a triangle).

Solution:-

We know that the sum of the angles of a triangle is 180o.

∴ ∠PQR + ∠QRP + ∠RPQ = 180o

= 105o+ 40o+ ∠RPQ = 180o

= 145o + ∠RPQ = 180o

= ∠RPQ = 180o– 1450

= ∠RPQ = 35o

Hence, the measures of ∠RPQ is 35o.

Steps of construction:

1. Draw a line segment PQ = 5 cm.

2. At point P, draw a ray L to making an angle of 105o i.e. ∠LPQ = 35o.

3. At point Q, draw a ray M to making an angle of 40o i.e. ∠MQP = 105o.

4. Now the two rays PL and QM intersect at the point R.

Then, ΔPQR is the required triangle.

3. Examine whether you can construct ΔDEF such that EF = 7.2 cm, m∠E = 110° and m∠F = 80°. Justify your answer.

Solution:-

From the question it is given that,

EF = 7.2 cm

∠E = 110o

∠F = 80o

Now we have to check whether it is possible to construct ΔDEF from the given values.

We know that the sum of the angles of a triangle is 180o.

Then,

∠D + ∠E + ∠F = 180o

∠D + 110o+ 80o= 180o

∠D + 190o = 180o

∠D = 180o– 1900

∠D = -10o

We may observe that the sum of two angles is 190o is greater than 180o. So, it is not possible to construct a triangle.