ANSWER
It is given that ABCD is a cyclic quadrilateral
(∠B−∠D)=60
o
......(1)
(∠B+∠D)=180
o
.....(2)
By adding both the equations
∠B−∠D+∠B−∠D=60
o
+180
o
So we get
2∠B=240
o
By division
∠B=120
o
By substituting equation it in equation (1)
(∠B−∠D)=60
o
120
o
−∠D=60
o
On further calculation
∠D=120
o
−60
o
by subtraction
∠D=60
o
Therefore, the smaller of the two angles ∠D=60
o
.
Explanation: