The correct equation is cos a= DE/DF.
Relationship between triangles
The cos represents the division between the side adjacent to the angle and the triangle's hypotenuse.
The sin represents the division between the opposite side to the angle and the triangle's hypotenuse.
As the triangles are similar, we can analyze the angles of triangle ABC with the sides of triangle DEF, so that:
Cos a = DE/DF
Sin a = EF/DF
Cos c = EF/DF
Sin c = DE/DF
So the correct answer is option C, cos a= DE/DF.
Learn more about relationship between triangles in: brainly.com/question/95152
