If two triangles have their sides proportional, the triangles will be equiangular and will have those angles equal which the corresponding sides subtend.
Proof
Construct on the second triangle's base a triangle equiangular with
the first (I.23); by VI.4 its other sides are determined by the
proportion, and by I.8 (SSS) it coincides with the second triangle.
Therefore the second triangle is equiangular with the first.