Set your calculator to DEGREES! Failing to do this is the WORST possible way you could possibly fail your Geometry/trig final. By default, TI-83s,83 Pluses, and TI-84s are set to radians. Therefore, when you enter:
tan45=
You will NOT get "1". You will get the answer in radians, not degrees.
To change it, go to "Mode", scroll down, and select "Degrees" instead of radians.
That is all.
-Shmads