Pi/2 radians different from 90 deg?

[Mike T.]

The difference is results obtained using degrees and radians seems to go back a long way, as on some older models (sin(cos(-1)) is 0 using degrees but when using radians the result is small but not quite zero.

I've tested this on an actual HP34C, HP10C and HP11C and all three return exactly the same result. Further testing using a simulator gives the same results for the HP31, HP32, HP33 and HP67 so I suspect that all these series of similar models behave the same way.

Interestingly the even older HP21 and HP25 both return zero when using degrees and radians.

It seems counter intuitive but I suspect that as the value of Pi is an approximation, the later models are technically more accurate...