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Draw an isosceles trapezium ABCD.

Join the diagonal BD and AC in the isosceles trapezium ABCD.

By drawing the diagonals inside the trapezium we can see triangles \[\Delta ADC\]and \[\Delta BCD\] are formed.

We know we can prove equal any angles or sides of two triangles if they are congruent.

Congruency theorem is the method used to prove sides or angles equal in the case of triangles.

In \[\Delta ADC\]and \[\Delta BCD\]

We know opposite sides are equal in an Isosceles trapezium.

AD = BC

We know the diagonals are equal in an Isosceles trapezium.

AC = BD

We know the side DC is common in both the triangles implies,

DC = DC

So, we can say that by (side-side-side) SSS congruency the two triangles are congruent.

\[\Delta ADC\cong \Delta BCD\] (SSS postulate)

\[\angle D=\angle C\] (Congruency property)