NCERT Solutions
Class 9 Maths
Constructions

Ex.11.1 Q.2
Construct an angle of 450 at the initial point of a given ray and justify the construction.
The below given steps will be followed to construct an angle of 450.
1. Take the given ray PQ. Draw an arc of some radius taking point P as its centre, which intersects PQ at R.
2. Taking R as centre and with the same radius as before, draw an arc intersecting the previously drawn arc at S.
3. Taking S as centre and with the same radius as before, draw an arc intersecting the arc at T.
4. Taking S and T as centre, draw an arc of same radius to intersect each other at U.
5. From R and V, draw arcs with radius more than RV/2 to intersect each other at W.
Join PW.
PW is the required ray making 450 with PQ.
Justification of Construction:
We can justify the construction, if we can prove ∠WPQ = 450.
For this, join PS and PT.
We have, ∠SPQ = ∠TPS = 600.
In 3 and 4 steps of this construction, PU was drawn as the bisector of ∠TPS.
So, ∠UPS = ∠TPS ÷ 2 = 600 ÷ 2 = 300
Also, ∠UPQ = ∠SPQ + ∠UPS
In step (vi) of this construction, PW was constructed as the bisector of ∠UPQ.
So, ∠WPQ = ∠UPQ ÷ 2 = 900 ÷ 2 = 450