Like the HP-20S, the HP-21S is also an algebraic calculator. “Untested” (??!). As per RSKEY, “There are two main differences between these calculators (the HP-20S and the HP-21S). First, scientific functions on the HP-20S, such as hyperbolic functions, unit conversions, or number base calculations are replaced with advanced statistical functions on the HP-21S; second, the set of built-in programs differs between the two machines.
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