As shown in the figure, in the quadrilateral ABCD and the quadrilateral a1b1c1d1, if AB = A1B1, BC = b1c1, CD = c1d1, Da = d1a1, are the two quadrilateral congruent 1. XiaoCong's idea: connect diagonal AC and a1c1 respectively. If AC = a1c1, then The two quadrangles are congruent. According to Xiao Cong's idea, write the reasoning process 2. The teacher said that under the condition that the four sides of the quadrilateral are equal, Xiao Cong Add a condition - diagonal AC = a1c1, you can show congruence Please add another condition (except diagonal) to explain the congruence of the two quadrangles and write down your thinking process

As shown in the figure, in the quadrilateral ABCD and the quadrilateral a1b1c1d1, if AB = A1B1, BC = b1c1, CD = c1d1, Da = d1a1, are the two quadrilateral congruent 1. XiaoCong's idea: connect diagonal AC and a1c1 respectively. If AC = a1c1, then The two quadrangles are congruent. According to Xiao Cong's idea, write the reasoning process 2. The teacher said that under the condition that the four sides of the quadrilateral are equal, Xiao Cong Add a condition - diagonal AC = a1c1, you can show congruence Please add another condition (except diagonal) to explain the congruence of the two quadrangles and write down your thinking process

Condition ∠ B = ∠ B1
{AB=A1B1
∠B=∠B1
BC=B1C1}
The ∧ ABC is all equal to ∧ a1b1c1 (edge)
∴AC=A1C1
The following solution is the same as (1)

Let AB = A1B1, BC = b1c1, CD = c1d1 in convex quadrilateral ABCD and convex quadrilateral a1b1c1d1,
Let AB = A1B1, BC = b1c1, CD = c1d1, Da = d1a1 in the convex quadrilateral ABCD and the convex quadrilateral a1b1c1d1. If ∠ a ＞ A1, the proof is: ∠ B ∠ C1, ∠ D

Connect BD, b1d1, ∠ a ＞ A1, so BD > b1d1, ∠ C ＞ C1
If ∠ B = ∠ B1, then AC = a1c1, ∠ d = ∠ D1, the sum of internal angles of quadrilateral will be more than 360 degrees, so ∠ B