My calculator displays 'NAN'; what does this mean?

[NEHA]

ok
i used the calculator to evaluate:
arc cos(tan45degrees)
now my answer came out to ..in the calculator : NAN what does this mean?

 

[tkhunny]

NAN means "Not a Number".

Most likely, due to rounding of the specific implementations of the tangent and arccosine functions in your calculator, the actual result is some very tiny Pure Imaginary Number - really close to the zero (0) you were expecting.

 

[skeeter]

... or, you may have your calculator in radian mode rather than degree mode.