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From: jlg@cochiti.lanl.gov (Jim Giles)
Newsgroups: comp.edu,sci.math,sci.misc
Subject: Cosine of Pi/2.0  (was: Subtle Math Questions)
Message-ID: <22614@lanl.gov>
Date: 25 Apr 91 15:41:37 GMT
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mjo@ttardis.UUCP (Mike O'Connor) writes:

> Well...  on an HP-11 or 15, you can take the cosine of pi/2 and get
> a number that is not zero.  It's rather annoying.

In 'radians' mode:
The calculator is just being honest.  There is no representable
number which is exactly equal to pi/2.  This means that there is
no representable number for which the cosine is exactly zero.
This is one of the first tests I make of a calculator: if it gives
zero for cosine of _anything_ that can be represented, it probably
has a bad cosine function.

On the other hand, if your calculator has a 'degrees' mode for the
trig functions, you _should_ get zero for the cosine of 90.0 degrees.
Similarly for 'grads' mode.

J. Giles