Determine, by virtue of which sign of equality of triangles, triangles ABC and CDA are equal, if AD = BC, AB = DC.

Knowing all three signs of equality of triangles, we can immediately say that triangles are equal according to the third sign of equality of a triangle – “on three equal sides”.

But in the task, only two sides of one triangle are equal to two sides of another triangle.

AD = BC, AB = DC.

But if you draw a drawing of a triangle, then they will have one side, AC is common, and points A and D each belong to its own triangle.

It turns out that the triangles have two equal sides according to this condition, and the third side, the AC they have in common, means. And it turns out equality on the third basis – the equality of the three parties.