Geometry
Congruence
Say when two figures are congruent, read a congruence statement in the right order, and use it to find matching sides and angles.
Two figures are congruent when one is an exact copy of the other: lay one on top of the other and they cover it perfectly. Sliding, turning or flipping a shape does not change it, so a shape stays congruent to itself however it is moved about.
The order of the letters is not decoration
Writing 'triangle ABC is congruent to triangle PQR' says A matches P, B matches Q and C matches R. That is what lets you read off the parts without seeing a picture: AB matches PQ, angle B matches angle Q, and so on. Writing the letters in the wrong order says something different and usually something false.
Worked example
Triangle ABC is congruent to triangle PQR. AC is 12 inches and angle B is 50°. What else do you know?
Line up the letters: A with P, B with Q, C with R.
Everything else follows from this one step, so it is worth writing out rather than doing in your head.
Try it together
Now one together. Triangle ABC is congruent to triangle DEF. Angle A is 55°, angle B is 65°, and DE is 8 inches long.
Nothing here needs a drawing. Match the letters up, then read the answers off.
1.Which side of triangle ABC matches DE? Type the two letters.
Have a go
Have a go on your own: triangle ABC is congruent to triangle PQR, and angle B is 48°. How big is angle Q, in degrees?
Hint: B is the second letter of the first triangle.
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.