Answer:

Step-by-step explanation:
First of all, let us do a construction in the given question image.
Let us connect the points E and C.
Please refer to the image attached in the answer area.
<u>Step 1:</u>
EC is drawn in the diagram.
Now, consider the triangles △EGC and △EGB.
1. Side BG = GC (G is the point on perpendicular <em>bisector </em>of AC)
2.
(G is the point on <em>perpendicular </em>bisector of AC)
3. Line EG is common to both the triangles.
So,from SAS (Side Angle Side) congruence i.e. two sides and angle between them are equal in two triangles.
△EGC ≅ △EGB
<u>Step 2:</u>
Now, we know that △EGC ≅ △EGB, we can say that the corresponding sides CE and BE must be equal to each other i.e. CE
BE .
And we are already given that BC
BE.
In
:
EB ≅ EC ≅ BC that means
is an <em>equilateral triangle</em>.
Hence, every angle of
must be equal to
.
∠BEC = 60°.
<u>Step 3:</u>
Let us find ∠ECD= ?
ABCD is a square, so 

<u></u>
<u>Step 4:</u>
We know that ABCD is a square, so side BC = CD.
We have proved in the previous step that BC = EC
in
, two sides are equal so it is isosceles triangle.
One angle ∠ECD= 
As it is an isosceles triangle, the other two angles will be equal.
Let them be equal to
.
The sum of all 3 angles of a triangle is equal to 

OR
we can say 
Now, from the diagram, we can see the following:

