TAN and ATAN functions ...

[Guest]

(a) From a formula: Y = TAN (X)

(b) if X=2.0 (in Radian) that's equivalent to 115 degrees

(c) Then => Y = tan (2) = tan (115 degrees) = -2.18

(d) Now I can reverse the formula to X = ATAN (Y)

(e) But, X=ATAN (-2.18) = -1.14; NOT 2.0 that I expected it to return -- see
statement (b)

Why is it so?

Thanks

 

[David Biddulph]

(a) From a formula: Y = TAN (X)

(b) if X=2.0 (in Radian) that's equivalent to 115 degrees

(c) Then => Y = tan (2) = tan (115 degrees) = -2.18

(d) Now I can reverse the formula to X = ATAN (Y)

(e) But, X=ATAN (-2.18) = -1.14; NOT 2.0 that I expected it to return --
see
statement (b)

Why is it so?

From the help:
ATAN returns a value in the range -pi/2 to +pi/2.

You need to remind yourself that all the trig functions will wrap round at
180 degree intervals.