Congruence and Similarity

Same shape, same size

• Two shapes are congruent when neither their form nor their measurements differ in any way
• Turning one round, flipping it over or sliding it somewhere else leaves it congruent, since a rotation, a reflection and a translation all move a shape without resizing it
• Enlarging a shape does change its size, so an enlargement is never congruent to the original unless the scale factor is 1
• To show two shapes are congruent, match up the sides and show corresponding lengths are equal, then do the same for the angles
• The two shapes need not face the same way, so match corresponding parts rather than comparing them where they sit

• Tracing paper settles it quickly on a diagram drawn to scale