the fx-39 say no more!this was a real trooper of a machine back in
1978 when it
came out. similar in look but somewhat improved over the fx-29.

the 39 had an *inv* button which means a
lot more could be accommodated on the keyboard (was this the first casio
with an *inv* button?). parentheses are also
here, not present on the earlier 29, and to a limited extent
implied multiply.

casio had also figured out a neat trick was to share
the *exp* button with *pi*. when entering
numbers *exp* would enter the exponent, when
operating with numbers the same button would recall pi.

another novelty
was inverting * hms* display. a decimal hour value could be converted back to
hours, minutes and seconds. displayed in the, now familiar, casio way with
a small upper digit degree sign.

an undocumented feature was that the
operator keys (+, -, *, /) implemented combinations, permutations and
polar/rectangular conversion when prefixed with the *inv* button exactly like the later fx-120. i have no
idea why this was never documented. perhaps they forgot to put the legends
on the front panel.