Question

A student solved the following problem and made an error:

Answer 1

All the steps were correct except the final statement. The mistake was in Line 6.

Line 6 triangle ABC is congruent to triangle EFD by SAS.

This does not follow. The SAS postulate states that if two sides and the included angle of one triangle is congruent to two sides and the included angle of another triangle. The student only proved that one side of the triangle (AC) is congruent to the side of another triangle (EF) .