casio fx-39

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.