The figure shows the two lines QU and ST intersects at point R and formed into two triangles.
Since,
Line QU = QR + UR
Line ST = SR + TR
We can say that the two corresponding sides of the triangle proportional and similar.
Angle QRS and Angle URT are pairs of vertical angles. Therefore, under the rule of vertical angles, they must be equal.
Since we've found out from the given figure that the Two Sides of the Triangle are proportional and Angle QRS and Angle URT are equal. The triangles are similar under the SAS (Side-Angle-Side) Postulate.
