NCERT Solutions
Class 10 Maths
Constructions

Ex.11.2 Q.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.